I think it's interesting that the unit test with the WebTestClient now passes but the manual calls exhibit the bug. When I originally reported this issue both the same unit test and manual tests exhibited the same problem. And were both fixed with the original fix.
This has been fixed and should be shortly available in Spring Framework and Spring Boot SNAPSHOTs. This has been introduced in #21783 for Reactor Netty and Tomcat. Using Jetty as a server in your sample works as expected.