Firestick 4K Max - Major Issues

[tmsrxzar]

compare to a copy of the 1080p video in the new log without pgs subs, ffmpeg -i (infile) -map 0:v -map 0:a -c copy (outfile).mkv

This actually played flawlessly, so I'm guessing it was all about the subtitles type. Any possible fix for that (maybe by not allowing subtitles extraction on the fly in the playback settings)? Or should I just avoid pgs subs?

i simply avoid pgs subs, i may be abnormal but i usually only use forced in my own language, downloaded from *somewhere on the internet* as a separate SRT next to the video
you can tinker with the settings or try conversion software but usually they need OCR to be converted which is a lot more effort than just searching google

I don't think the log for this playback is of any help but if it is I'll share it. Looking at the playback information from the dashboard, it said that the playback wasn't on direct play but on remuxing, claiming that the streams were compatible with the device but the container wasn't so it is repackaging it on the fly. Is this expected behaviour?

honestly, i see this from time to time and i don't know where it comes from, never investigated as it plays fine when being remuxed

you may try alternative clients if you wish, you will "likely" get better results; kodi+jellyfin addon can do pgs and a lot more than generic android for example

(otherwise you'd be looking at different devices entirely which i don't want to pitch or sell to you since you're not asking about that stuff)