Make sure to use Get Surface Type and NOT to Break Hit Result from the result of the Line Trace. If you Break the structure to get at the Phys Material and then get the surface type from that, it will always return default. (There is of course a way to resolve this but I am unsure exactly how to do so with the blueprinted Line Trace. If it were two objects which are defined, you can have their collisions return the Surface Material).
TLDR: Make sure to use the GetSurfaceType function that takes in a HitResult and returns the user defined enum for Surface Material type.