I have had gyro's do this as well, and it did worry me. However, when I just said "YOLO", and put it in the air, everything was fine and it was stable. Not saying that is how your experience is going to go, and I would use extreme caution putting it in the air (be ready to flip out of that mode), but may want to try it.
Basically, from my experience, the gyro is expecting a movement, the helicopter is not moving, so it keeps adding pitch just like the tail does in HH mode.