EncoderHttpMessageWriter computes and sets the content-length for Mono values since 5.0.5, see
SPR-16542, but also checks to make sure there is no "transfer-encoding" header set already. Since Reactor Netty sets "transfer-encoding: chunked" by default int the beginning, this became and issue, after the recent changes in 5.1.1, see to wrap the headers of the underlying server, in turn requiring a workaround in ReactorServerHttpResponse. SPR-17250,
While the check for whether transfer-encoding is set isn't an issue in itself, it should not be necessary. It's not a header set by the application typically. Arguably it's better to remove this check in order to be able to remove the workaround in ReactorServerHttpResponse, and essentially leave the decision for transfer-encoding to Reactor Netty.