Unlike WebMvcAutoConfiguration.WebMvcAutoConfigurationAdapter (a WebMvcConfigurer), WebFluxAutoConfiguration.WebFluxConfig (a WebFluxConfigurer) is unordered. This means that it implicitly has lowest precedence and manes that it's impossible to guarantee that any other WebFluxConfigurer will go after it. Our WebMvcConfigurer is @Order(0). We should consider applying the same order to our WebFluxConfiguer as well.
See https://github.com/spring-projects/spring-boot/issues/12389 for details of why the ordering was configured on the MVC side. If we agree to make this change, we'll need to think about when we want to make it. Theoretically, changing the ordering will break an application that's relying on the current ordering but I'm not sure how likely that is. FWIW, we made the MVC change early in 2.0.x's life (2.0.1).