Since I tackled the same issue here my bp of the cube object which only gets moved in z axis. One important thing to change is in the BP_motioncontroller > ReleaseActor. There the check if an object is attached or not needs to be bypassed. Otherwise the dropped event is not getting sent.
Here the changed BP_motioncontroller network:
Also, I was not really able to get the motion conroller reference through a new variable. I was only able to use the Pickup event “Attach To” to get it. Any idea why that is?