Landscape will only return "DefaultPhysicalMaterial" when run in editor world. In game time it will return the proper physical material.
Found in 4.19 CL# 4033788, 4.20 Preview 3 CL# 4135216 , 4.21 CL# 4136834
The returned physical material will show as the default while in editor mode but in game time the correct physical material will be displayed
In both editor and game time it would return the same physical material set for each layer
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-60798 in the post.