Have also had this problem for over a year. Tried Cambium support, but they kept trying to tell me the problem was with the switch interface connected to the near side PTP650. I could not reason with them that their analysis was flawed as there was no packet loss through the radio link to the customer. I finally gave up.
My guess is that it is a firmware issue. Since it does not affect my customer, I am willing to live with it although I am disappointed with Cambium's support.