[Tracked controllers] Issue with momentum loss on dropping objects

I'm having an issue with interacting with objects. If I try to pick up and toss them, they freeze, and then fall with gravity, rather than continue it's path due to montum. I've tested this in a basic scene just to make sure there's nothing from my game interfering.

I have the ViveController, with a trigger collider attach to the target cube with a rigid body, via a fixed joint.
When I "let go" the breakforce is set to 0. AFAIK the target cube should continue with it's momentum. If there is no gravity the cube is completely still. It's momentum is being destroyed somehow.
What is going wrong?