Using World Position as texture coords in mesh decal doesn’t appear to work - it just shows up white. It does work in a decal material applied to a deferred decal actor, but not when applied to a mesh.
Repro: Follow the mesh decal steps here: Using Mesh Decals | Unreal Engine Documentation - except use WorldPosition.RG as UV coordinates on whatever texture you’re using.
Workaround: It does seem to work per-vertex, so you can throw WorldPosition through a Vertex Interpolator node, and that works fine for most cases.