I have several clients that have this same issue where one or more APs become a Master also. Here is what I mean when I login to what I know to be the Master dashboard with a static IP, one or more APs are disconnected which are DHCP. After a bit of hunting for the other APs IP I am able to login to those disconnected APs and one of them are also a Master.
My only recourse, that I know of, to put them back on to the Master that I want is to remove the disconnected APs in the Master dashboard and then reset them.
It's as if the communication between the master that I chose and the other APs are lost for a little bit (hour or so?) one of the others take the Master role. Once the communication is restored, the role doesn't reverse and the original master is still the Master while the other APs is also a Master.
I also see this same behavior. You actually end up with duplicate IPs on the network which I think is what causes the "disconnection" from the new Master. Only fix I have found is to not give the master a static address. That, however, is not ideal for most my installations where I want to externally manage the unleashed network.
I haven't noticed but not likely. I have two clients and each have 4 Unleashed R600, all on 1 POE managed switch. The switch is on a UPS and it has never lost power. But each client on has had this happened to them.