The Halo problem is exactly why I liked having the alpha channel and background image option.

In P5 I would render over my selected background image, but I would also get the Alpha info without the image being there.  This allowed me to mask out my foreground subjects and add things to the background or to adjust colours and such without effecting the foreground and vica-versa.

Since I rendered over what was mainly going to be in the background I could negate the halo effect in most situations.

Oh well, if this isn't a bug, but instead a "desired effect", then I just have to get used to a new workflow.  Which I have been employing anyway for the most part.