A lot of "bad" or unallowed collections get added to TMDB. These get deleted once reported but could be on the site for days or years before they are found. Once the item is deleted on TMDB, Kometa will throw an error that the TMDB ID can not be found. This happens daily until the TMDB cache expires/refreshes.

My preference would be to have Kometa throw a warning not an error and just delete the collection.