Layer's preview weight does not have an affect on the Preview Material for Landscape Layer Blend nodes

Layer's preview weight does not have an affect on the Preview Material for Landscape Layer Blend nodes.

Repro Rate: 3/3

Tested this in //UE4/Release-4.27 @ CL#18319896 and the preview window did get the material applied, it is a regression!

1. Open a new blank BP project with Starter Content
2. In the Content Browser, create and open a new Material
3. Right-Click and add in a Landscape Layer Blender node
4. In Details, add a new layer to the node
5. Set the Preview Weight to 1.0 for the layer
6. Connect the output of the blend node to the Base Color for the new Material
7. Right-Click in empty space, and add in a Texture Sample node
8. In Details, set the texture to "T_Fire_Tiled_D"
9. Set RGB to the Layer None
10. Apply & Save

Results: Material Preview displays a grey orb with no signs of the texture.
Expected: Material preview should be textured with the given Texture Sample due to the weight on the Landscape Layer Blend