I don't have any answers and I am sorry that I can't devote time to this right now.
I am wondering if once a message has been read, if one party deletes both the inbox and outbox version of the message thread, that it gets cleared from the system, including the 2nd party's PMs.
That my guess. If two of you want to participate in an experiment to see if I'm right, please feel free.
I am talking about the forum PMs. I don't know if this happens on the main site, but either way, that one is a much better PM system, and I suggest using it.
Sorry for the inconvenience.