HeightmapShape gets KinematicBody stuck, unlike BoxShape or ConcavePolygonShape

[Zylann]

Godot 3.2.3
Windows 10 64 bits
Bullet physics

From initial report at Zylann/godot_heightmap_plugin#217
Heightmap shapes seem to cause trouble with kinematic bodies in particular (using sphere shape). When move_and_slide is used, bodies tend to just stop their motion abruptly and remain stuck. This does not happen if the ground uses a BoxShape or ConcavePolygonShape. RigidBodies seem to be fine.
So far the only thing that fixes this is to increase the safe margin parameter of the KinematicBody to 0.01, the default being a minuscule 0.001. I find it strange that this collision shape causes trouble by default, while the others apparently dont.

Video showing the problem:
2020-11-17 19-40-27.zip

MRP:
HeightmapKinematic.zip
Play main scene, wait until the ball lands, hold right arrow. It starts to accelerate, and then suddenly stops.

Odd note: in this MRP the ball's Z coordinate is near 0. But if it starts at 10, it will get stuck later, in a different place.