OnTriggerEnter / Stay not called with non-convex collider

I have an extremely specific case where neither OnTriggerEnter or OnTriggerStay is called.

I have a disabled box collider fully inside of a non-convex mesh collider. The box collider is set to be a trigger and has a kinematic rigid body attached.

When I enable the box collider neither OnTriggerEnter nor OnTriggerStay get called on that non-convex mesh collider.

If I change the mesh to convex it works.
If the box collider is not fully contained in the mesh it also works. (If it's big enough so that some of it is sticking out a side of the mesh collider)

It's only a problem when the mesh is non-convex and the box collider is fully contained within it when enabled.

I'm using a pretty old version of Unity - 2017.4.6. Was wondering if anyone knows if this is an issue that was fixed and if so in what version, or if it's an open known issue or what.

I don't see any references to this in the bug database or through normal searches.

 

Anyone?