The message SE438 is related to commitment reduction which got introduced with OSS note 580595 - Value-based commitment reduction for service PReqs
Message SE319 is also implmented with an OSS note 1133175, details and reasons are explained in KBA 1904679 - Service entry sheet with zero quantity
I think your commitment did not go up again with your PO deletion.
Why don't you just delete the old PR and create a new one?