When using the preview camera feature it turns out as well as culling the game objects, this also culls the editor gizmos from the preview camera, which makes the editor hard to use in this mode.
To get around this problem we simply disable frustum culling for GLOBAL portal_mode objects when in preview camera mode. This could be a bit slower in an editor scene with lots of gizmos but is the simplest way of solving the problem.
lawnjelly