We don't have control over the connection at the web framework level, so it sounds like a Reactor Netty issue. For a quick experiment, try adding "spring-boot-starter-tomcat" to see how WebFlux on Tomcat behaves. I suspect it will work as expected since it works on Spring MVC.
After adding "spring-boot-starter-tomcat" it works as expected. It has good behavior on handling HTTP 1.0 and the Connection header. The link quoted by Brian is very useful. It might be the Reactor Netty's issue. Thx