The real fix is in CL 27191295 from epic mainline (not been released in any public UE version yet).
It’s a one line change in ULandscapeComponent::UpdatePhysicalMaterialTasks()
Find this line in LandscapeEdit.cpp:
// Clear the renderable physical material properties as we don’t need them :
and add this line above it:
PhysicalMaterialHash = Hash;