I’m not sure, but it looks like when importing complex UCX-meshes, the engine simplifies them by removing some vertexes. I think there is some limitation on the complexity of UCX-meshes, because they are, in fact, converted into elements of the Convex collision. Therefore, as a variant of one of the solutions, I recommend splitting UCX meshes into simpler primitives that have a maximum of 8 vertices (cubes, parallelepipeds, prisms or cylinders or cones). As a result, you should get sets of UCX-meshes like this: UCX_MainMeshName_00, UCX_MainMeshName_01, UCX_MainMeshName_02, etc.