ASS subtitles?
Plex is working on adding ASS subtitle support to the Android clients.
Long thread focused on Android TV: Nvidia SHIELD/Android app SSA/ASS format anime subtitles + transcoding problems
Based on comments in the thread, the work will carry over into the mobile clients as well.
In the meantime…
Do you have Auto Adjust Quality enabled in the Plex Android app (Settings → Quality)? If so, try disabling it.
If not ASS, what subtitle format?
FYI, you can check PMS version via Authorized Devices: https://app.plex.tv/desktop/#!/settings/devices/pms.
