Currently TomcatHeadersAdapter directly writes to the MimeHeaders map, as part of the performance improvements in #21783. We are also applying specific changes on the response in org.springframework.http.server.reactive.TomcatHttpHandlerAdapter.TomcatServerHttpResponse for the Content-Length and Content-Type response headers.
The problem is we're still writing those headers to the underlying map, which means those headers are written no matter what happens to the response, even if other parts of Tomcat codebase are removing them using the expected API.
We should never add those headers to the map in the first place.