I think what you mean is called "yaw" in aviation ("gieren" in German). A rotation around the vertical airplane axis from the view of the pilot.
I like this behavior of the Sylphyo and I believe that it was implemented on purpose as it is.
Due to the magnetic field sensor, a "roll" gesture started in a horizontal Sylphyo position gets less effective, the more you angle the instrument downwards. Near the final vertical position no more rolling is measurable. This can be musically useful, but it also means, that starting in a steep downwards position, which can make sense in a visual performance, would not have any roll possibility unless you move the Sylphyo up. I assume that Aodyo implemented the "yaw" movement as a "second roll". While the Sylphyo is held steeply downwards, you can yaw/(roll) it with a pendulum motion. Futher up you can move on to the normal roll gesture.
I see this implementation as simplification for new users and for players who prefer just a few technical possibilities over loads of features.
But personally I think that yaw should be an independent parameter. We can always set yaw and roll to the same controller number and this could even be the default setting (like it is now). It is a good idea to set defaults for easiest playing, but when we start to tweak, all motions should be usable in any way we want.
Delicate: In a more horizontal position, "yaw" collides with the "compass" parameter. Tricky stuff ...
I think that the movement of the Sylphyo is not perfectly worked out yet, but since it also depends on features still in beta state, I would prefer to discuss this in a separate thread.