HISTORY time travel table “validto” field not updating

HANA History time travel table. In this I am facing some issue while updating the records in history table,  “$validto$” field is not updating with “COMMIT_ID”.  We have tried multiple time out of that many time $validto$ field not updated. But sometime $validto$ filed got updated with proper COMMIT_ID. Any suggestion why “$validto$” field value inconsistent.?