Okay, I've just figured out what is happening — not trying to discredit you as I was playing around right after the first person's reply (but still big thanks!).
The good news is that it's easy to resolve, the bad news is that it's slightly annoying and seems like it's a bug — see video. I emptied Trash before the screen record.
*If you don't want to watch the video, try collapsing / expanding different notebooks until it appears.
