Possible bug with spring cloud gateway

Description: Currently, I have a service using Spring Cloud Gateway and Spring Boot WebFlux. Its global error handler captures all failed requests (including body and headers) sent to the final API and uses that data to trigger another process within the service. Previously, the service ran on Spring Cloud Gateway 4.1.x, Spring Boot WebFlux 3.3.x, and Spring Context 6.1.19, and it worked fine: when a request to the final API failed, the global error handler would capture every modification applied by Spring Cloud Gateway (for example, headers added with the AddRequestHeader filter). Now, after updating to Spring Cloud Gateway 4.2.x and Spring Boot WebFlux 3.4.x, the global error handler isn’t capturing the modified request—it only sees the original request without the Gateway’s changes. Internally, we use a filter responsible for capturing the response using ServerWebExchange and WebFilterChain and throw and exception when the response code is related to an error.

Start with a minimal Spring Cloud Gateway/WebFlux reproduction comparing the 4.1.x/3.3.x and 4.2.x/3.4.x combinations. Trace the global error handler's ServerWebExchange/WebFilterChain path and the AddRequestHeader filter; done means isolating whether the modified request is lost and documenting a reproducible failing case or regression location.