Okay, I’ve managed to solve both my issues that I originally listed in my post: that being Plex crashing when playing a video with embedded VOBSUBs and Plex refusing to play anything with a DTS track (including DTS tracks untouched from MakeMKV) on my Android TV.
The fix was pretty simple, and I can’t believe I didn’t try it earlier because it’s what I always try first when an app misbehaves on my phone.
All I had to go to when long press on the Plex app icon whilst in the homescreen, click on app info, force stop the app then clear data before relaunching the app and signing in again. And that’s it. Somehow the exact videos I was having problems with are now working just fine (all direct-playing).
This was the behaviour I experienced before the latest few Plex updates so I’m not sure what went wrong or what clearing the data did to fix it but hey it works.
But unfortunately, it’s not all roses as in doing so I discovered a new issue (which I suspect is related to this one relayed to me by @pshanew above) whereby if I try to play a video that has embedded VOBSUBs and at the same time require audio transcoding (multi-channel aac 
), then Plex would just crash and restart. If I turn off the subs, then the video launches fine, and audio is transcoded no problem.
I’d appreciate if we could hear an update on this issue since I checked the post that’s linked and there hasn’t been an update from Plex staff since that initial reply 15 days ago. Thanks all for your attempts to help!
EDIT: Upon further digging, it appears the reason why DTS tracks failed to play on Plex on my Android TV is because I had turned on the ‘Refresh Rate Switching’ option. What? How? Why? I’m pretty confused by this one and I’m pretty sure it’s replicable too. Take a video with a DTS track, try to play it on a Sony Android TV set with Plex and you should see the same behaviour I’m seeing. I don’t think the ‘Refresh Rate Switching’ option ever did anything on Sony Android TVs anyways so I’ve left that off but would appreciate an explanation into this.
