For a repro open the LandscapeMap in the attached project. It contains a landscape with 3x3 components, with the center one having been retopologized (i.e. using a landscape mesh collision component).
If you simulate as it is you’ll see the all spheres falling on the landscape as expected. Now stop the simulation and change the Collision Mip Level of the landscape to 1. If you simulate again you will see the sphere over the center landscape component going through the landscape.
Could you guys have a look at it?