If I add titles to new TV episodes which are not matched, the next time I do a “scan library files” the titles are erased even though they are showing the “locked” icon. The “sort title” is not erased but retains what I enter. I think this behaviour is new.
No, it’s not new.
Never add metadata manually to “unmatched” items.
Always “match” them first to the Personal Media metadata agent.
(in the case of tv shows, you can only match the show as a whole, not any episodes individually).