Hi Simon,
I have finished investigating this, and ignore both points above. The quick answer is that this specific setup won’t work. (Ie something with a purely vertical cable attached to a kinematic or simulated body which is moving towards the floor when the distance is less than the cable). The longer answer is that the cable cannot push back on the kinematic or dynamic body, nor can it get out of the way - so it gets more and more compressed until something gives - in this case it punches through the ground.
This cable component system sits essentially outside of core physics - so it uses a different system which is faster, but less stable than the normal collision detection and solvers (where the constraints and contacts are solved together iteratively, as opposed to independently in this one).
If you can let me know a bit more about the specific use case I may be able to suggest something a bit different.
Best
Geoff