I see a problem in mikrotik very strange, do not know if it's bug, I have a
PPPoE server, in version 6.9, all customers with ubnt AirOS dialing the PPPoE,
the problem is this, when a client turns off the radio, he disconnects the
mikrotik and 20 seconds after tipping another PPPoE client, whenever it
happens, a client turns off the radio with 20 seconds after tipping another
PPPoE client without having anything to do. following image
this happens numerous times in the day, a client logs out the PPPoE server and
PPPoE drops another client together.

all versions tested... same problem..
tested all versions including 6.9, only problem is when the squeegee server pppoe over bridges, then BCP bug in pppoe-server .. hopefully solution .. problem happens several times a day ..

Just out of curiosity, have you connected a few devices of other flavors besides UBNT to see if they behave that way? What are your PPP settings?