After a recent version Plex on my PS5 as well as my Samsung Smart TV refused to play Original quality at all, it just stayed black on screen and stuck on 0:00, whereas in the dashboard it showed the video progressing but I think the latter is just visual and not really representative of the actual progress of the video.
Dropping the quality from 8.1Mbps (Original) to 8Mbps works instantly, it also works perfectly on mobile even on Original.
Reverting to Version 1.25.5.549 fixed it, and I seem to be the only one with this issue as I see no such complaints anywhere else.