TempDB Transaction log not releasing space

Every time I complete a transaction in tempdb, the temp log grows, however the log doesn't seem to release the space once the transaction has completed.The log usage percent is 43.4% but seems to increase in around 4% increments. When I look for open transactions (sys.dm_exec_sessions), there are none. Yet the log_reuse_wait_desc in sys.databases shows 'ACTIVE_TRANSACTION'. When I query the sys.dm_tran_active_transactions DMV, it shows only work table. I'm unsure of how I can release this space from the tempdb transaction log. I've just run a manual checkpoint and that seems to free up the space so I'm not sure as to why the space isn't being freed up on it's own