I can confirm this behaviour, I have an E4.5 and a M10 FHD and both make frequently a restart, when I hit power off. I think the second time it works normally. I can't find any rules behind, it seems randomly for me. Possibly it is only after they have been running for a while? Testing right now I can't reproduce it. But I always wait and check weather the device is really powered off.
I think the bigger problem is, even when charging the M10 in powered off status, I come back after some time and the M10 is running again without having it restarted manually. If I had the screensaver disabled possibly the screen was on for hours.