checking again earlier, it just so happens that all of them were mkv with embedded subs. One thing I'm noticing also is that all these files are transcoding instead of direct play (even though on my Plex server, when watching the same media on the Android app and the same phone for instance, would've direct played instead of transcode) - when I use the media player client on macOS however, they are direct playing as expected and it doesn't seem that subtitle desync issue is present.
If you don't mind answering these questions:
1. Is the "criteria" to avoiding the need to transcode different between Plex/Jellyfin? Jellyfin seems to transcode my media better than my Plex server, but on my Plex server I almost never need transcoding on any devices of mine so I've disabled it completely since idt my server is "built" for transcoding.
2. I assume, the web UI/player will always require transcoding and cannot direct play? Are the Android and iOS app facing the same "limitation" perhaps bcos they're not actual "apps" but rather, they're a wrapper of the same web UI?
3. On Plex, there's a setting where I can just completely disable transcoding so that it simply errors out if a media cannot be played directly by a device - seeing that transcoding on Jellyfin causes these subtitle desync issues, is there a way for me to do the same?
