That is normal behavior for most of the smaller Ultimates...

Due to the torque of the motor, and the angle of the wings and elevators, one side is stalling faster than the other.

You can INCREASE the throws at high rates, to cause the wings to stall faster and more evenly during high rate pulls, and reduce the throws at low rates a bit at a time, until you find the point that the loops stay round with the stick fully back.

I've owned a slew of Ultimates and biplanes and most show this characterstic with certain throw positions.