This is another behavior I’ve seen with newer recordings. Seems to be mostly on recordings/live TV from HD channels. You may want to check my post below, which fixes this for recordings played back later. It won’t fix live TV though.
If you find a solution to make Plex work as is, I’ll be interested in it but I wasn’t able to.
https://forums.plex.tv/t/bug-closed-captions-display-corruption-multi-platform/904341
