I'm observing that after REQUEST_SUBSCRIPTION it also sends a RESPONSE frame with the COMPLETE flag set. This is out of spec obviously but might break a less lenient implementation.
This is caused by ClientReactiveSocket passing a Px.just<Payload>() which emits a onComplete() that is handled by RemoteSender (created in handleStreamResponse()) here: https://github.com/ReactiveSocket/reactivesocket-java/blob/b8fd58aaafec022bacc7a9c5f214d720f3535dd1/reactivesocket-core/src/main/java/io/reactivesocket/internal/RemoteSender.java#L162