Convex MeshCollider using cylinder geometry doesn't match original mesh in Unity 2018.3.6

I'd made a scene in Unity 2018.2.17 that had a cylinder with 80 sides that had a convex MeshCollider component. The green MeshCollider matched the Model Mesh perfectly vertex for vertex.

But when I use the same scene or model in Unity 2018.3.6, the green MeshCollider has been reduced, producing unwanted triangular zigzags (see attached screenshot file), which creates an unwanted wobble when the cylinder rolls down an inclined plane.

Am I missing some setting in Unity 2018.3.6 to prevent this polygon reduction?
I've read somewhere that convex meshes are limited to 256 triangles - is this still the case in 2018.3.6?

Any help with this is much appreciated!