Virtual joysticks in QGC leads to ever increasing altitude

I’m trying to get QGC running with my DJI quadcopter drone using Rosetta Drone 2.

If I activate the virtual joysticks in QGC and activate the checkmark for the automatic centering for throttle, then the drone continues to climb in height.

I debugged the behavior in the code a bit and found that QGC sends “manual control” messages when the virtual joysticks are activated. A thrust of 500 is transmitted if the setting is active that the throttle is centered. This is transmitted to the controller in RD 2 as a positive value for the Z-axis (approx. 1.92 …) and thereby the drone continues to climb.

If I deactivate the automatic centering, a value of 0 is correctly transmitted for the thrust (no change in height). However, I can no longer fly downwards with the thumbpad.

I don’t know whether the problem is with RD2 or with QGC. On the one hand, the MAVLink protocol specifies a value range from -1000 to +1000 as valid. So the middle should be 0. Thus, QGC would be out of specification. On the other hand, I have already tested QGC with other firmwares (PX4) with the virtual joysticks and everything worked here. It could of course be that PX4 implements the MAVLink specification incorrectly as well, but I think that is unlikely.

What would be the best way to get the behavior right?