I too am having this issue. After I updated to 11.1. I started receiving this message and confirmed this with uptime and have physically seen it reboot. It is only happening on one of my machines though. it is an intel server with dual xeon processors and 8GB of ecc ram. I use it to store my photography. I thought it was hardware so I disassembled the whole thing and cleaned it, re-assembled it with different hard disks, and did a fresh install. it didn't happen for about a week, It started rebooting again a few days ago.
It's on the same power supply as the other two freenas boxes. Neither are having problems. Both are on 11.1 one is an old dell workstation, and the other is an old hp board I slapped together with some non ecc ram, in a case I cut a hole in the front of and tack welded to a botched together drive cage. just to put my old drives in when I rebuilt the intel server. which seems counter intuitive. They have no jails or transmission, and all running 11.1-u2
It's not on a ups, but it would stand to reason they would all reboot, wouldn't you think?