It's probably the gain... if you have any revo mixing in HH mode, you're heli will spin, not just rotate 45 degrees and stop. In HH mode, the gyro sees the revo mix as a yaw command from you, so you'd actually have to give a corresponding rudder input to cancel the revo mix and keep it from rotating.

I've already been there and done that. I set up a heli assuming the TX had no revo mix in the heli program by default... turns out it had 20% by default... in HH mode the heli would want to spin, in rate mode it was fine... I finally decided to go into the revo mix menu and check it... doh!