I’d say you might have had a snapshot still holding the deleted data when you first deleted the cache. I don’t use time shift for my backups but I’d assume it uses the same kind of incremental snapshot as btrbk. Which means that, until the next backup date, it will hold onto the previous state of the system, preventing it from truly deleting the file.

You may also have some balance issues, having way more metadata allocation than needed. Try running a balance and see if it changes something.