I am having a similar issue. Here is my setup:
tvOS version: 15.6 (stable) | iOS version 16.0 (beta)
Plex app version: 8.7 (stable) | iOS app version: 8.8 (beta)
Plex server version (Windows): 1.28.2.6151 (beta, I believe)
Whenever I play videos with VTT subtitle files, they start as being ‘off’, then when I turn them on I get two sets of subtitles, one on top of the other. When I then turn them off again, the rear (and larger) set of subtitles remains and keeps going as if they were still turned on, despite them being turned off. These are all documented in the attached screenshots from the tvOS version.
EDIT: This also happens on the iOS version, but doesn’t happen on the Android app on our Sony smart TV, or on the desktop app for Mac. I have not tested the desktop app for Windows.
