I still haven't seen anywhere reporting what, exactly, was the issue, and why would using the "true Administrator" account on Windows (or using Linux) avoid the issue.

My own uninformed guess, given that the "true Administrator" account avoids it, would be some sort of security mitigation, for instance flushing the branch predictor state when entering or exiting the kernel unless the current user is SYSTEM or Administrator.

If you have a hot path with a lot of branch mispredictions that can impact performance. Could be as simple as Microsoft found a dumb if inside a tight loop.