Failed to process OIDC Back-Channel Logout and cannot customize OIDC Back Channel Logout related classes

OidcBackChannelLogoutAuthenticationProvider and related classes are protected final and doesn't accept custom classes because of next phases instanceof control. The main issue is that I'm using Keycloak, and when a back channel logout request is sent, Spring Boot can't process it. The log will be more descriptive:

DEBUG --- [0.0-8081-exec-1] .c.a.w.c.o.c.OidcBackChannelLogoutFilter : Failed to process OIDC Back-Channel Logout
org.springframework.security.oauth2.core.OAuth2AuthenticationException: An error occurred while attempting to decode the Jwt: Signed JWT rejected: Another algorithm expected, or no matching key(s) found
	at org.springframework.security.config.annotation.web.configurers.oauth2.client.OidcBackChannelLogoutAuthenticationProvider.decode(OidcBackChannelLogoutAuthenticationProvider.java:126)
	at org.springframework.security.config.annotation.web.configurers.oauth2.client.OidcBackChannelLogoutAuthenticationProvider.authenticate(OidcBackChannelLogoutAuthenticationProvider.java:103)
	at org.springframework.security.authentication.ProviderManager.authenticate(ProviderManager.java:182)
	at org.springframework.security.config.annotation.web.configurers.oauth2.client.OidcBackChannelLogoutFilter.doFilterInternal(OidcBackChannelLogoutFilter.java:104)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.springframework.security.web.ObservationFilterChainDecorator$ObservationFilter.wrapFilter(ObservationFilterChainDecorator.java:241)
	at org.springframework.security.web.ObservationFilterChainDecorator$ObservationFilter.doFilter(ObservationFilterChainDecorator.java:228)
	at org.springframework.security.web.ObservationFilterChainDecorator$VirtualFilterChain.doFilter(ObservationFilterChainDecorator.java:138)
	at org.springframework.security.web.header.HeaderWriterFilter.doHeadersAfter(HeaderWriterFilter.java:90)
	at org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:75)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.springframework.security.web.ObservationFilterChainDecorator$ObservationFilter.wrapFilter(ObservationFilterChainDecorator.java:241)
	at org.springframework.security.web.ObservationFilterChainDecorator$ObservationFilter.doFilter(ObservationFilterChainDecorator.java:228)
	at org.springframework.security.web.ObservationFilterChainDecorator$VirtualFilterChain.doFilter(ObservationFilterChainDecorator.java:138)
	at org.springframework.security.web.context.SecurityContextHolderFilter.doFilter(SecurityContextHolderFilter.java:82)
	at org.springframework.security.web.context.SecurityContextHolderFilter.doFilter(SecurityContextHolderFilter.java:69)
	at org.springframework.security.web.ObservationFilterChainDecorator$ObservationFilter.wrapFilter(ObservationFilterChainDecorator.java:241)
	at org.springframework.security.web.ObservationFilterChainDecorator$ObservationFilter.doFilter(ObservationFilterChainDecorator.java:228)
	at org.springframework.security.web.ObservationFilterChainDecorator$VirtualFilterChain.doFilter(ObservationFilterChainDecorator.java:138)
	at org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:62)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.springframework.security.web.ObservationFilterChainDecorator$ObservationFilter.wrapFilter(ObservationFilterChainDecorator.java:241)
	at org.springframework.security.web.ObservationFilterChainDecorator$ObservationFilter.doFilter(ObservationFilterChainDecorator.java:228)
	at org.springframework.security.web.ObservationFilterChainDecorator$VirtualFilterChain.doFilter(ObservationFilterChainDecorator.java:138)
	at org.springframework.security.web.session.DisableEncodeUrlFilter.doFilterInternal(DisableEncodeUrlFilter.java:42)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.springframework.security.web.ObservationFilterChainDecorator$ObservationFilter.wrapFilter(ObservationFilterChainDecorator.java:241)
	at org.springframework.security.web.ObservationFilterChainDecorator$AroundFilterObservation$SimpleAroundFilterObservation.lambda$wrap$0(ObservationFilterChainDecorator.java:334)
	at org.springframework.security.web.ObservationFilterChainDecorator$ObservationFilter.doFilter(ObservationFilterChainDecorator.java:225)
	at org.springframework.security.web.ObservationFilterChainDecorator$VirtualFilterChain.doFilter(ObservationFilterChainDecorator.java:138)
	at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:233)
	at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:191)
	at org.springframework.web.filter.CompositeFilter$VirtualFilterChain.doFilter(CompositeFilter.java:113)
	at org.springframework.web.filter.ServletRequestPathFilter.doFilter(ServletRequestPathFilter.java:52)
	at org.springframework.web.filter.CompositeFilter$VirtualFilterChain.doFilter(CompositeFilter.java:113)
	at org.springframework.web.filter.CompositeFilter.doFilter(CompositeFilter.java:74)
	at org.springframework.security.config.annotation.web.configuration.WebSecurityConfiguration$CompositeFilterChainProxy.doFilter(WebSecurityConfiguration.java:319)
	at org.springframework.web.filter.CompositeFilter$VirtualFilterChain.doFilter(CompositeFilter.java:113)
	at org.springframework.web.servlet.handler.HandlerMappingIntrospector.lambda$createCacheFilter$4(HandlerMappingIntrospector.java:267)
	at org.springframework.web.filter.CompositeFilter$VirtualFilterChain.doFilter(CompositeFilter.java:113)
	at org.springframework.web.filter.CompositeFilter.doFilter(CompositeFilter.java:74)
	at org.springframework.security.config.annotation.web.configuration.WebMvcSecurityConfiguration$CompositeFilterChainProxy.doFilter(WebMvcSecurityConfiguration.java:240)
	at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:362)
	at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:278)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:162)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:138)
	at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:162)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:138)
	at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:162)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:138)
	at org.springframework.web.filter.ServerHttpObservationFilter.doFilterInternal(ServerHttpObservationFilter.java:110)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:162)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:138)
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:162)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:138)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:165)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:88)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:482)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:113)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:83)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:72)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:399)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:903)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1774)
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52)
	at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:973)
	at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:491)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:63)
	at java.base/java.lang.Thread.run(Thread.java:1474)
Caused by: org.springframework.security.oauth2.jwt.BadJwtException: An error occurred while attempting to decode the Jwt: Signed JWT rejected: Another algorithm expected, or no matching key(s) found
	at org.springframework.security.oauth2.jwt.NimbusJwtDecoder.createJwt(NimbusJwtDecoder.java:188)
	at org.springframework.security.oauth2.jwt.NimbusJwtDecoder.decode(NimbusJwtDecoder.java:142)
	at org.springframework.security.config.annotation.web.configurers.oauth2.client.OidcBackChannelLogoutAuthenticationProvider.decode(OidcBackChannelLogoutAuthenticationProvider.java:121)
	... 77 common frames omitted
Caused by: com.nimbusds.jose.proc.BadJOSEException: Signed JWT rejected: Another algorithm expected, or no matching key(s) found
	at com.nimbusds.jwt.proc.DefaultJWTProcessor.process(DefaultJWTProcessor.java:394)
	at com.nimbusds.jwt.proc.DefaultJWTProcessor.process(DefaultJWTProcessor.java:340)
	at org.springframework.security.oauth2.jwt.NimbusJwtDecoder.createJwt(NimbusJwtDecoder.java:162)
	... 79 common frames omitted

Related https://github.com/spring-projects/spring-boot/issues/48485
Spring Boot 3.5.8 and Spring Security 6.5.7

Start with OidcBackChannelLogoutAuthenticationProvider and OidcBackChannelLogoutFilter, using the Keycloak back-channel logout request and stack trace as the reproduction path. Read the related Spring Boot issue, then establish what prevents the request from being processed and what customization of the related classes is expected; done means the reported logout failure and customization limitation are addressed.