When you uncheck Multiple Directions on the player/npc script, it also hides the Frame Flipping dropdown, which doesn't seem like it should be hidden? It does still respects the setting whether it is hidden or not.
I was looking for that setting because a new character I made was walking backwards, and eventually I found it when checking Multiple Directions. I then set it to Right Mirrors Left, unchecked Multiple Directions again and it worked.
Comments
When "Multiple directions" is checked, it looks for _U, _L, _R and _D variants for each of the standard animations. If frame-flipping is checked, it'll only look for _U, _L and _D (or _U, _R and _D depending on what kind of frame-flipping you've chosen).