Add/delete/rename nothing directly in the Libraries KNOWN folders.
A number (##) just indicates that at one time, the folder WAS created by calibre. It may no longer be
active.
When you do some library metadata edit operations, that BOOKS number might change. Calibre does a 'Make New then delete the Old folder' transaction.
Something might have blocked the Delete portion, or your cloud put the old folder back
See if that book exists elsewhere (different #, with a slight title change).
Move the UNKNOWN folders to a SAFE area, then add those books back the traditional, calibre way