Hendrik:
Late to your thread.
You mention a calm evening. How fast were you descending and was it vertical?
There’s an aerodynamic phenomenon in helicopters and powered lift machines called “settling with power”.
Essentially, in a high rate of descent, the aircraft is descending in its own rotor/prop wash. The air flow through the rotor system is very unstable and can lead to large stability fluctuations. This could have been what you were seeing.
Early into the effect, yes you can power out of it, but once the effect is established, you won’t have enough power because more power only accelerates the phenomenon.
It’s easy to test - descend vertically in calm conditions. You will see the frame oscillating as the thrust varies between the props. If you see this, add forward or side stick and the air frame will fly out of the unstable air flow.