I too have seen this once or twice, but many revisions ago. And it was only adding the very most recently registered devices (maybe 2-4) to groups. But since then my fog server died and I have rebuilt adding a storage node and haven’t seen this happen in a while since keeping up with the revisions of the development. I actually forgot about it…
I will also check this again, although recently I registered I have registered quite a few new machines and created a few groups and not seen this happen. I’m on Debian 8, FOG 1.3.0