Did a bit more playing around last night to try and find a little more about this bug.
The pppoe connection will also drop when the first device connects to wlan1, any other devices that connect or disconnect to wlan1 will not drop the pppoe connection, as long as one device connected & has registration its fine.
It's only when the last device disconnects from wlan1 will the pppoe session will drop again.
ela002 mentioned in this post
He noticed this happens when the bridge the status of wlan changes from designated to disabled. pppoe then dropps.
Being this appears to of been around for a while (rolled back to v5.8 problem existed then also in the latest version 5.15) I will raise a support ticket in the next day
Found a fix, take ether1 out of the bridge (port bridged to modem) attach the pppoe session to ether1 and pppoe no longer dropps out. your also not bridging your local network to your isp