Subtitles not showing

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Roku is weird about pretty much all subs. If you have preferred language subs enabled by default, you might try turning that to OFF by default, starting to play the media, then selecting the proper subs. Other route is to try playing the media, turn subs off, then turn them on again. Embedded subs, if they are PGS, will 100% cause transcoding on the Roku platform.

You can find your logs in your JF instance without going to the OS. Go to Dashboard > Logs with an admin account and you'll see both Jellyfin logs as well as any ffmpeg logs for transcodes that occur.