Unable to configure Eureka port

We need to be able to run our services with dynamic ports in Amazon ECS. We have an internal Spring Boot starter which is able to successfully get the ECS port and set it on a new EurekaInstanceConfigBean bean.

However, the EurekaAutoServiceRegistration class is overriding this value.

Here are our log files. You can see that our starter is finding port 32768. However, you can also see where EurekaAutoServiceRegistration sets it afterward.

2020-09-09 16:18:58.021  INFO [,] 1 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8001 (http)
2020-09-09 16:18:58.987 DEBUG [,] 1 --- [           main] c.a.s.cloud.boot.EurekaAutoConfigure     : Received host port 32768 from ECS container metadata.
2020-09-09 16:18:58.988  INFO [,] 1 --- [           main] c.a.s.cloud.boot.EurekaAutoConfigure     : Setting non-secure port to 32768
2020-09-09 16:19:02.370  INFO [,] 1 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8001 (http) with context path ''
2020-09-09 16:19:02.375  INFO [,] 1 --- [           main] .s.c.n.e.s.EurekaAutoServiceRegistration : Updating port to 8001 

I believe the relevant code in Spring Cloud Eureka is this:
https://github.com/spring-cloud/spring-cloud-netflix/blob/master/spring-cloud-netflix-eureka-client/src/main/java/org/springframework/cloud/netflix/eureka/serviceregistry/EurekaAutoServiceRegistration.java#L139-L150

It seems that this registration should not be attempting to override an existing configuration. Also, since it running based on a, WebServerInitializedEvent, we can't use Spring Boot's configure-after controls.

Start with EurekaAutoServiceRegistration.java at the linked lines and trace how WebServerInitializedEvent leads to the port update. Reproduce the interaction with the custom EurekaInstanceConfigBean and verify that an existing configured port is not overwritten after the web server starts.