2D background panorama with unexpected order swapping?

Surrounding my terrain, I have a panorama of 2D mountains on the inside of a cylinder. Outside of that I have a 2D sky Material on a SM_SkySphere. For some reason, when the camera gets high enough, they swap order.

In this demonstration, I have two mountain cylinders inside a sky sphere. You can see the mountains are swapping order as the camera goes higher, before they both go behind the sky sphere (hide the skysphere and the mountains reappear). They aren’t disappearing, just swapping order, so I’ve ruled out a culling or bounds issue.

Is there a way to prevent this? Could it be a material setting? The materials are set to Unlit, Translucent.