Hmm, I have a suspicion how this happens...
I could more or less reproduce the issue:
2 Kamailio instances, synced with DMQ. Both store dialog_vars in a local database for performance.
While a dialog is ongoing, stop instance 2. This instance 2 probably has stored dialog information in it's local database.
End the dialog. Instance 1 propperly removes dialog_vars from it's local database and forgets about it.
Restart Instance 2.
No longer existing dialog data from instance 2 is getting DMQ synced to Instance 1. This dialog is sticking around on both instances and never expiring.
Mit freundlichen Grüssen
-Benoît Panizzon-