Tom, as a temp work-around, using the MeshImpostor seems to work. BoxImpostors also work, but not as well as MeshImpostors.
https://playground.babylonjs.com/#6QF3KP#11
I doubled the linearV’s to 2 and -2 for a little more effect.
Possibly, Ammo doesn’t allow non-uniform-scaling/elongation of sphere rigidBodies. Not sure.