The answer is to check Build Adjacency Buffer in each static mesh you import via the Static Mesh editor in Unreal. See:
https://forums.unrealengine.com/development-discussion/rendering/81933-enabling-pn-triangles-or-crack-free-displacement-causes-default-material-to-appear-on-certain-meshes
and
https://forums.unrealengine.com/development-discussion/content-creation/1792200-how-to-build-adjacency-buffer-for-many-meshes-at-the-same-time