I too have run into this problem on a number of devices. The client's supported codec/media format detection logic appears to be extremely pessimistic. To the point that the mere existence of certain audio track formats and subtitles can trigger transcoding, even when said tracks/subtitles aren't actively used. The same files played via MPV on a file share, i.e. no transcoding possible, play without issue.

I've tried tweaking both the clients and server to be less pessimistic, i.e. see if the active media elements are supported, but even then it really fights it. I think the best approach would be rewrite the clients to adopt a (try || transcode) strategy rather than (guess || transcode), but that looked to be a major piece of work.