We are experiencing this, too. We have Workfront set to time out after 30 minutes of inactivity, but there's no way to set a time for ProofHQ and they don't seem to be recognizing each other. The timing seems random as well.
It is a huge security risk not to be able to globally delete someone from proofs on which they are the creator. When you hire seasonal contractors, there must be a way to quickly delete them when their contract is up.