Mesh Collision and Convex destroying Mesh

Hey guys,

So having a bit of an issue with the Mesh Collider. Basically have a mesh, not overly complicated but complex enough that I need an accurate collision box. I have tried a few things, one was decimating the mesh down to below 255 tris and importing that as a collision mesh. However when I tick convex the mesh gets destroyed down to 3 Tris.

Then I made an even more primitive mesh using Box geometry, maybe 10-12 Boxes scaled and moved around in Blender to fit the outline of the mesh. Imported it and same thing, mesh gets dropped down to a few Tris as soon as I tick convex.

Finally I decided to import 1 Box as a test. 8 verts, 6 sides, 12 edges, 12 tris. Your basic standard box. When I tick convex on that the mesh gets decimated down to 3 Tris. What exactly is going on?

Thanks!