OK I get it now. It's a Cycles bug (feature?) if there's nothing in the background Cycles/SuperFly treats is as bring at infinity. Which blows out any visible queuing between objects with any object less than 1/255th of infinite distance ... with is of course still infinite.FireFly just ignores the background if there's no geometry there.

I've filed a bug report. For the meantime if you make sure the background it totally covered with anything, a scaled up square plane will do, that will make it work.

P.S. If you reply to the bug email I don't actually get those (probably so mail filter someplace). It's better to respond to the actual bug report. Them I'm sure to get it.