I started seeing this behviour in 6.5.0 and never seen this in 6.4.0 where all entries for the human task are deleted when task is marked as completed. Is this something new?
This behavior is from jBPM 6.0.0 release. This change in behavior because of the [JBPM-3543] Clean Tasks from DBMS when a Process ends - JBoss Issue Tracker