Status of all episodes updated at the same time

I have recently added a new TV programme to my library called Red Dwarf. When I watch an episode Plex marks every episode in the series as marked. If I watch half an episode it marks all episodes in the series as half watched. Essentially whatever status an episode is in, is copied to every other episode in the same series.

I’ve been using Plex for years and have loads of other TV programmes which all behave normally. The episodes are named “1x01 - Name.avi” and are stored in a separate folder for each season. I am running the latest version of Plex Server on a Mac Mini and using Roku, web client and ATV4.

Any ideas what would cause this?