Still not convinced it's power-based. My wife got the 27GN950 basically at release, and it's never had a black screen from the same circuit. Changing outlets, power extensions and keeping it on its own circuit didn't change the frequency of the issue at all either.
Well, I hope that it is not power-based so that it can be fixed by a firmware update, but from my own experience and how I fixed it for me - it is power-based.
From your case, it might be a lot of things. Maybe electrical interference which triggers black screens originates from somewhere "upstream" of the power grid and changing wall sockets does not affect the black screen issue. Or maybe when your wife used the monitor it wasn't at VRR mode or fps was higher than upper threshold, or vsync was on, all of these would disable the VRR and, as a consequence, there will be no black screens no matter how long one uses it.
Maybe somebody has a laboratory oscillograph with reference power source which can introduce artificial interference. That will be an ultimate test.
UPDATE: did not notice that you spoke for GN model, not GP; maybe LG cheapped out on the power adapted for the GP950. If it is not difficult for you, share some photos of the GN's and GP's power bricks. That would be interesting.