I have some whole databases as favourites. When they’re removed as favourites, they nevertheless reappear as favourites on re-launching the app.
If the other person was right in thinking that some (as yet unidentified) characteristics of certain files were preventing permanent removal, then I was guessing that I may well have those types of file within those favourite databases, and perhaps they were preventing the whole database-favourite from being removed. Of course I don’t know if that’s right (but there again, we don’t know what’s preventing the removal of favourites anyway).