I’ve been looking at a number of threads reporting this same issue. It does not appear that Plex has acknowledged this as an issue yet. I just created a new Library for Photos and this bug is affecting all videos stored in this new library.
Edit: I finally found a thread where it is acknowledged by Plex. From what I can tell this has been an known and reported issue off and on for more than 3 years, but more recently has started affecting more users. I’m surprised it has been so long without a fix…hopefully they can actually fix this.