When changing the Material of a CompleCollision during PIE, the change is not applied to the PhysicsMaterial.

If changing the material during PIE, the content of the material will be reflected immediately, but the physics material will not. Once the Mesh (Complex Collision Mesh) is cleared and reset, it will be reflected correctly. 

1. Open the attached project.
2. Play in editor.
3. Walk on the green and brown facing blocks (SM_Mesh_A). Walking on the green area will log "Grass" and walking on the brown area will log "Wood".
4. Open Mesh(Col_ComplexCollisionMesh_A) and replace index0 and 1 in Material Slots. 
5. Again walk on the green and brown facing blocks. 

Result:

Walking on the green area will log "Grass" and walking on the brown area will log "Wood".

Expected:

Walking on the green area will log "Wood" and walking on the brown area will log "Grass".