Mikrotik PPPoE Problem Bug

I see a problem in mikrotik very strange, do not know if it’s bug, I have a PPPoE server, in version 6.5, 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.
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