I have “Hide bottom bar during review” set to always, but when I move my mouse off of Anki, it briefly focuses Anki when it passes the edge of the application, and then the bottom bar stays appearing. The workaround is to click on my desktop, click back onto Anki, and then quickly click back onto my desktop. Then I Alt+Tab the focus back to Anki to use my spacebar to flip the card and continue my reviews with the keyboard.
The easiest way to replicate is to start from the home screen, select a deck. Wait until the bottom bar disappears after a second, and then move your mouse off of Anki through the side. Then it’ll reappear and stay there.