Without a replay, I cannot ensure what happened. As stated above, it says When a face-up monster you control is flipped face-down while you control no face-up monsters:
It says When which can indeed miss timing. Not only that but there is an additional condition here while you control no face-up monsters
There are many different situations where it wouldn’t occur. Bug reports need exact steps to reproduce not an assumption of what should happen. Give me the exact scenario and steps to reproduce it.