This is actually an ANNOYING ASS BUG in Plex. It has nothing to do with watched status, and the fact that you fix it by changing watched status is just probably part of the bug. I have this all the time and sometimes exiting out and back in will even fix it. I have no idea what actually causes it, but this is absolutely a bug.
See here BUG: Randomly chapter thumbnails disappearing using new player on shield
If you’re not using a ShieldTV then that simply confirms the bug isn’t specific to the ShieldTV client.
