Landscape shader doesn't work with macro textures

I’m working on a landscape and after much experimentation (some evidently incorrect YouTube tutorials are to blame) I’ve been able to set up a landscape in a way Unreal is somewhat happy with.

Assembling my shader however I soon ran into a big problem - I have macro normal and diffuse maps from World Machine which I want to blend over my tiling normal and diffuse maps. With the macro maps plugged in, the shader breaks completely and all I get is a checkerboard all over my landscape.

My material uses texture objects plugged into WorldCoordinate3Way expressions, which are then plugged into Layer Blends. These have the first texture set to weighted blend and the rest set to height blend, into which I have height maps arriving courtesy of standard texture samples. Using a mixture of weightedblend and height blend was the only way I could find to reduce black areas appearing on my landscape. The standard suggested fix of using one layer set to alpha blend did absolutely nothing on my scene. With one set to weightedblend the black areas are reduced greatly but still present. This in itself is a really nasty bug which needs to be fixed urgently.

My macro textures are just textures samples with a landscapeCoords nodes generating the UVs, which are the same as the landscape size.(4033) The normals are blended with a BlendedAngleCorrectedNormals and the diff is blended with a MultiplyAdd.

With either (or both) of these plugged in, the shader breaks completely.

Am I missing something or is this another bug in the landscape system?