If an HTTP2 client sets the SETTINGS_MAX_FRAME_SIZE it essentially has no effect because the frame size used to sent to the client is based on the Undertow setBufferSize() option as well.

This is not ideal - even if the server is using a smaller buffer size it should still send frames up to the max frame size set by the client - it should use more chained buffers in this case.

As I said, you can get around this by setting a larger setBufferSize() but this has overhead implications - especially if the client wants  to use really large frames - up to 16MB.