First off, I’m by no means a programmer, so this is all conjecture and made up terms in the areas I lack formal knowledge, I ask that those who know better forgive and/or correct me.
I believe that nodes do not have access to the actual render time sequence of things and so are really just a bunch of post production filters. With that being so, the problem isn’t that foreground objects can’t bleed over farther back ones, but the exact opposite is the problem. Meaning that the background object isn’t rendered enough to show behind the foreground when that is blurred.
I suppose the best/only way to work around this would be to render the background objects on a separate layer so they get rendered more completely, and then composting the blurred foreground on top of that.
anyways, that’s the problem as I understand it, and the workaround I thought of, but I as I said, I haven’t tested to see if that composite trick would be easily pulled off.