Just to update, AC-3200, had a failure today on alpha2. Lost all external connectivity seemingly from all devices both wifi and ethernet. Not just DNS but no connectivity. I couldn't reach my upstream cable modem from the router either. Nothing in the error log. STP is disabled.; No indications of any error on the cable modem.
Similar symptoms to what I'd seen before disabling STP, this time it look a little longer, but perhaps it's just luck
Intrigued to see if there's any effect with newer gpl code and/or wifi drivers as I believe this issue affects core fw and merlin.