I DID fix my issues with shadows, it had to do with my lighting settings, specifically the ‘Dynamic Shadow Distance’ on the directional-light that is the Sun in Ultra Dynamic Sky. Since the default is 40k units, and I was zoomed-out all the time, the dynamic shadows were simply too-far away to be drawn in.
To help resolve, I found that setting, as well as the cascaded shadow-maps have an impact here.
And yes, it’s the underlying landscape that appears the generate the shadow. The ‘casts shadow’ setting on the heightmesh itself is seemingly non-functional (?). I toggle it on and off in various places and can’t see a difference…
here are my current settings:
and backing up a bit to something more than 40k units, you can see the shadow of the cube is gone, but at least the far-shadows on the landscape still remain:
That far part of the screen on the top-left is ~10km away in world-units. Still has shadowing 
EDIT: totally forgot, sorry! But you also want to enable ‘hidden shadow’ on your landscape. as, at least the way i am doing it, the landscape is never rendered, only used for the projection into the RVT and for collision, but since the heightmesh won’t make shadows, the landscape still needs be drawn in the main-pass for lighting.
