Thank you tomsc for the confirmation.
Some of the clients have updated to the latest agent version and software version, some have not. I have not discerned the pattern. We will manually update the remaining problem clients as they do not appear to want to update automatically. After that, I will allow traffic to bits.avcdn.net and see if traffic continues to originate from all clients or just the mirror server.
In the meantime, I have opened a case with tech support and provided logs for both the mirror server and one of the problem PCs.