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!
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
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-149647 in the post.