Wenaptic this isn't a problem of the bullet behavior, it is a result of the way you've set your system. The collision polygons of both your sprites are too small and after a while the sprites miss each other.
I' am sure that there is a better way to achieve the result you're after but I can't quite envision it. A quick fix using your setup would be to reset the Sprite's collision polygons back to full size and add a small delay before each angle of motion change. A wait action of 0.2 seconds looks ok for your bullet's current speed.