TempDB Version Store cleanup
(Note: this isn't a 100% certain answer, and I don't have references, so let me know if you can prove otherwise.)

The version store can only clear versions based on the oldest active transaction within the entire instance, to support the use of transaction-level snapshot isolation across multiple databases simultaneously. That simply wouldn't work if "old" versions within one database were cleared out in the middle of another database's snapshot transaction.

So, if there's a very old open transaction, the version store will not be able to clear until that transaction either commits or rolls back.