SOLVED: Jellyfin adds Specials to every single TV show

[generic]

Scanning the TV Shows library finds all shows, seasons and episodes correctly and adds all the art from themoviedb, thetvdb and fanart. But after some time (hours, at the latest next day) all shows suddenly get the Specials folder added (with no episodes inside). 
I have only 1 show with Specials and that is found correctly and during initial scan. 2 or 3 shows don't get the Specials added at all, but that is because thetvdb doesn't list any specials for them.

I can do a "Refresh metadata" on those specials or on the entire TV show and they disappear. But without fail they always return after some time. Is there some sort of cronjob running or maybe some sort of re-scan after server reboot?

Deleted and re-scanned the library a couple of times, always the same. Logs don't show anything interesting. Is there a debug setting maybe?

Edit: One more thing I just realized: I'm using Jellyfin for Kodi for multiple instances. Those specials are not synced to Kodi. That 1 show with the correct specials is though. The phantom specials are only appearing when using a browser and Jellyfin Media Player.

Local install on Ubuntu 22.04.04 LTS Server
Jellyfin 10.9.7 via Debian repo for Jammy

[TheDreadPirate]

Are you using NFOs for metadata? Or do you have missing episode fetcher enabled?

[generic]

Not using NFOs. I found solutions mentioning the missing episode fetcher and turned it off. There's an option on the server and the client, if I remember correctly?

Just checked:
'Display specials within seasons they aired in' is off
'Display missing episodes within seasons' is off

[TheDreadPirate]

There is a setting in the library settings where it lists missing episodes as a metadata provider (this is part of the TVDB plugin). Other users who turned that on, but wanted to get rid of it, had to completely delete their library and re-create it without missing episode fetcher enabled.

[generic]

Unfortunately deleting and re-scanning the library didn't help. However, apt purge and a clean re-install did help. Skipped adding the libraries during the initial setup wizard and configured all settings first, including all plugins. Only then added the libraries.

Appreciate your help!