I noticed that both my AP11 and AP11D reboot at random times, or at least i get notifications telling me this.
I use two different power sources: my AP11 is powerd via PoE+ injector and both AP11D's are powerd via a PoE+ capable switch with enough powerbudget .
However if you investigate connected clients then it may very well be that the connection time is 20hrs while 5 hrs ago i received an offline notification. At the same time of the notification the network is reachable and the other 2 AP's appear online.
I have checked another site with AP15's and there i couldn't find this behaviour.