Hmm, this is a slightly tricky one. There are two angles: the angle of motion of the Bullet behavior, and the object angle (the visible angle). The 'Rotate' behavior doesn't have any awareness of other behaviors - it only sets the object angle.
What you want is for the rotate behavior to set the angle of motion, but not the object angle. This isn't easy to support because it means the rotate behavior has to gain awareness of other behaviors and modify them, and this isn't really in line with how C2 behaviors tend to work.
What really is the bug report is setting the object angle shouldn't affect the angle of motion when bullet's 'Set angle' is set to 'No'. In this case the object angle and angle of motion should work independently. Previously, if you ever set the object angle, C2 would also update the angle of motion. This has been fixed for the next build so setting the bullet's angle only affects the object angle and not the angle of motion. However, the result is still not what you wanted: the object moves continually to the right, since the angle of motion is always 0, while the object appears to rotate.
There is also one exception: the angle of motion is still set to the object angle on the first tick of the behavior. This is to preserve compatibility with existing projects like Space Blaster, where many objects are spawned then 'set angle' used to modify the bullet angle. The bullets would fly off at the wrong angle unless they initially take the object angle as their first angle of motion.
Hopefully that's clear... however, even with the fix, you'll still need to use a second hidden object or manually set the angle of motion with events to get the effect you want.