I've been having a problem with certain MKVs direct streamed using Plex to a Roku- I started a thread in Plex support about the issue. However, I recently tested with the MKV files on a USB stick and it exhibits the same behavior, which means the problem is between the MKV file and Roku OS itself, not Plex.
The Roku will abandon playback shortly after starting to play the MKV, anywhere from 13 seconds in to a couple minutes in. I did many, many tests with different MKV export parameters and narrowed the issue down to only occurring when a PGS (forced only) subtitles stream is present in the MKV.
In reviewing some of the Plex logs that I posted in their support thread, it appears that the player thinks it's getting a signal from a subtitle stream that the title is over so it stops playback.
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3/13/2021 13:42:18.557 [num:email:ID] INFO (PlaybackMgr) VideoPlaybackController::UpdateSubtitleSuppression: state=finished trackName=mkv/off suppressCaptions=true
3/13/2021 13:42:18.560 [num:email:ID] INFO (PlaybackMgr) Player state changed to finished (hasAd=false)
Titles where I've experienced this issue include Lady Bird, Blindspotting, Once Upon a time in Hollywood 4K. The first two are Lionsgate with FPL but the third isn't. All of them work fine when I uncheck the PGS (forced only) subtitles stream when creating the MKV.
Anyone else have this issue with MakeMKV and Roku or have any ideas about the true cause?