Can you go into Android BubbleUPnP, Now Playing tab, tap floating cast button, 3 dot menu on your Yamaha > Info.
Does the "Audio" section lists "WAV" ?
That list of supported formats is built from the media mime-types the renderer declares to support.
If WAV is not listed but L16 is, that's why BubbleUPnP Server do the WAV -> L16 conversion (which does not incur a loss in audio quality).
And it is very well possible that this Yamaha does not declare supporting WAV while it does in reality.