When you do "Set bullet angle 180", make sure that, in the same event, you are creating the bullet before you set the bullets angle, so that Construct 2 knows which bullet to pick to change the angle of.
So lets say "Space" shoots a bullet, it should look like:
"Space" is pressed
Is Mirrored
Create object "Bullet"
Set Bullet angle to 180
"Space" is pressed
Is NOT Mirrored
Create object "Bullet"
Set Bullet angle to 0
[You could make this into sub events to be efficient but it's not important ]
What this does, is when you create the "Bullet", Construct 2 then focuses on that particular bullet and WILL NOT affect other bullets.
If you did this as an event instead:
"Space" is pressed
Is Mirrored
Set Bullet angle to 180
Create object "Bullet"
Then it will first detect all bullets and set their angle to 180, THEN create a new bullet and not even set its angle!
Hope this helps.