Yeah that aspect of bullet can cause some confusion. What you can do in this case is add two sub event on space pressed like
if player is mirrored -> bullet set angle 180
if player not mirrored -> bullet set angle 0
In my game , i had a character with bow and arrow and i had to handle bow mirroring, bow angle , arrow mirroring and arrow angle depending on mouse position and if player was mirrored or not. Took 3 days to figure everything out :)