Render selected seems to have unselected objects contribute black to the alpha channel when i use it in interactive mode, as it should.
But when I use it in regular mode, it is as if I have simply deleted the unselected objects, aka, the alpha gets all wrong, and selected stuff that is hidden behind unselected objects is seen in the render. And I just disovered this today when I have a tight deadline! Halp!
Alright! After some snooping I found out what was wrong. Admin can move this to resolved, but it is still interesting IMO:
I had “selection” selected in 3ds max native rendering dialog, under “common”. When I deselect it corona behaves as you’d expect, flawlessly. 
But it is still interesting that corona behaves differently in interactive mode and in “regular” mode. How come?
This is probably because 3ds Max’s “area to render: selected” is unsupported by Corona’s IR. Basically it works pretty much like hiding everything but the selected object. So it seems that everything you described is expected, but I will leave the topic here so that Ondra/someone else can decide whether it should be closed.
Maybe we should have a warning message that IR won’t work with “render: selected”?
or… we can make it supported now that Corona area to render: selected is supported by IR 
stand by for fix