Gotcha, and I see now that this was the issue you originally reported. The few times this has happened to me, Plex did not save playback status, and I wasn't able to select subtitles or chapters.
Have you tried to mount without cache? Just for testing.
Alright, I'll check that next, as well as trying with jellyfin... Maybe it is a plex issue.. Since @Animosity022 took a look at my logs, and all seems to be "normal"..
What client are you using to access your Plex server and are you also using CloudFlare?
I've been experiencing the same issues using the Plex for Windows app and some TV's. Plex Web and consoles were fine.
Issues with Plex for Windows app:
Unable to resume from a previous session - Video starts from scratch
Pause and resume after a minute or so - Video starts from scratch
Select chapters - Video starts from scratch
Unable to seek - Input seems to be ignored by the app
Randomly ends the video
Issues with some TVs:
Browse media but unable to play, loads for ages then errors out
Unable to turn off subtitles mid stream
Randomly ends the video
It didn't matter if it's Direct play or Transcode.
My issue turned out to be my CloudFlare settings - By changing the Caching Level (Caching > Configuration) from "standard" to "No query string" or "Ignore query string" has resolved bulk of the issues.
Not sure which is best but I'm sticking with "No query string"
I have had one user say that a video did randomly end twice which I'm still looking in to.
I do not have any page rules to bypass cache and Argo Tiered Cache on.
Alright.. So yes, I use CF, I now changed to No query string, lets see if that fixes the issue!
But it actually sounds exactly like that...
My Clients where Desktop (win) but also Android on my Phone
I have a rule like plex.mydomain.com/video/* that has bypass set for the cache level...
But maybe its to restrictive...