When completing the task Task Handler may create a separate thread for this. The session reference is passed to that thread, but the thread does not have a persistence context associated with it. That what I think is going on, but I’m not 100% positive. I have been fighting similar issue a few month back and ended up rewriting the handler to execute the work in the same thread. Also, you might want to look at the set of properties of the handler which may control this behavior.
HTH
Vladimir