Firestick client looks to direct play files, but android phone / tablet transcodes?

[argentsquest]

When i've been watching videos on our main TV / firestick, it looks to direct stream the files as expected but if I start watching via my phone or tablet, it starts transcoding instead. If I disable the option to allow transcoding then it simply won't play on these devices. I have VLC amongst other players installed and can stream directly with zero playback issues, so is there a way to add any 'missing' codecs to the android app short of forcing it to play VLC via the external player option? I'm running CasaOS on an i5-8500T with 16gb RAM and GPU enabled - disabling GPU didn't make a difference..

For reference, the files in question are 1080p HEVC SDR, with 5.1 AAC audio.