It is indicated in the Bullet behavior's manual article :
Set angle of motion
Set the angle the bullet is currently moving at, in degrees. Note: when the speed is 0, the angle of motion is always 0 and cannot be changed, since there is no motion. Therefore setting the angle of motion then the speed does not work if the object is stopped. Instead, set the speed first and then the angle of motion.
No motion, no angle of motion, speed must be set first.
When the speed is set to 0, there is no more angle of motion.
-> Sprite: Set Bullet speed to Pause ? 0 : 500
-> Sprite: Set Bullet angle of motion to 90 degrees
Does the job of keeping the sprite moving toward the bottom.