Delete behaviour on Sessions doesnt seem to always work

I have an app that associates certain objects to the session of the current user (with an after-create event).  What should happen is that when a session expires, these objects that are linked to it, should be deleted.  I put delete behaviour on this association that deletes the objects when the session object is deleted.    Only what I noticed is that when the user logs out, these objects are indeed deleted.  But when I check the database, I can find many instances of this object that are still there but have no session connected to them. They are always supposed to be initially connected to the session (with an after create event on entity level). So I would expect that when the user's session expires (i.e. the session object is deleted), these objects should also be deleted. But this doesn't always seem to happen.    Any idea how this is possible? 
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