Hi! I had the same problem. When I activated the ALT HOLD mode, I had a constant sink rate. No matter if I activated the ALT HOLD mode via the hotspot or via my honeycomb throttle. I also did not know why. Then I found out the following.
When I activate the ALT HOLD mode, the aircraft holds its altitude. Then I select the V/S MODE on the honeycomb. Now when I turn the knob I can set a sink or climb rate. Of course I can set the sink rate back to 0.
Perhaps this V/S MODE is not intended and has not been implemented. There is also no possibility to activate this mode with the mouse in the cockpit. But in the background there are the parameters for the V/S mode. And you can change these parameters e.g. via the honeycomb throttle.