It did turn out to be a database issue even though the DBRepair tool couldn’t find a fault. I’ve since deleted and remade my TV LIbrary and it’s now working again.
Unfortunately, the issue is still present for me in Plex for Windows.
I’ve tried to replicate the issue anywhere else but have so far been unsuccessful. This means the issue is isolated to just the Plex for Windows app. I’m glad it’s at least not DB related, as I ran the DBRepair tool and it didn’t fix it. Then I found this thread!