WPO is incorrectly generated on materials using Material Attributes as input to Landscape Layer Blend node

This comes from this UDN : [Link Removed]

It occurs with this setup : 

[Image Removed]

, where Set Material Attributes expressions act as inputs to the Landscape Layer Blend material expression.

But it doesn't occur in this configuration : 

[Image Removed]

, where colors are input to the Landscape Layer Blend material expression and the resulting color is input into a Set Material Attributes node. 

In the former case, the generated shader contains this : 

float3 GetMaterialWorldPositionOffsetRaw(FMaterialVertexParameters Parameters)
{
    float2 Local9 = Parameters.TexCoords[3].xy;
    float4 Local10 = Texture2DSampleLevel(Material.Texture2D_0,GetMaterialSharedSampler(Material.Texture2D_0Sampler,View.LandscapeWeightmapSampler), Local9 ,0.00000000);
    float Local11 = dot(Local10,Material.PreshaderBuffer[1]);
    float3 Local12 = lerp(((float3)0.00000000),float3(0.00000000,0.00000000,0.00000000),Local11);
    return Local12;;
} 

, which matches what the Landscape Layer Blend expression does : sampling the weightmap with the View.LandscapeWeightmapSampler, and then masking the result with some channel mask coming from a material param to only retain the channel that corresponds to the weightmap, and finally, lerping 2 values (0 and 0, here!) with the sampled weight as the blend factor. 

Use the attached project, open the map there and inspect the generated shaders in RenderDoc (using r.DumpShaderDebugInfo=1, r.Shaders.Optimize=0 and r.Shaders.Symbols=1 and running with -dx11 helps!)

The Basepass shader for the landscape materials with paint on it have a custom WPO, even though the landscape material used doesn't actually affect WPO.