When running in windowed fullscreen mode in a standalone build r.vsync 0 is not respected and the game always runs at a locked 60fps. When the game doesn't have focus, for instance when you press Win, the framerate becomes uncapped. Similarly, if the game is run in fullscreen mode instead of windowed fullscreen, the framerate is uncapped.
Follow reproduction steps here:
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-82798 in the post.