If triangles don’t appear it’s likely a Mesh issue where you’ve got your Normals flipped (you have the back face pointing out, google it to see the fixes).
As mentioned, use PreciseConvexDecomposition instead of Default for the collisions.
Also I’ve mentioned in quite a few of those posts that you can see the DecompositionGeometry of MeshParts and Unions (there’s a tool in the Studio settings, or there are plugins available) to show you exactly how they are shaped physically versus how they are rendered.