Sorry for this question but I've been banging my head about it.
I have a purple box. I put it on a floor (plane) with a reflective material. I render and I got my reflections.
Now I add a background with the same material, put a composite tag on the plane, check composite background.
And the reflection is gone.
I want a simpe reflection on a seamless white background.
How do I accomplish this?
I think you may have something else in your scene that might be interfering with the reflections. I tried what you did and I was able to get the seamless floor effect. I've upload my file 2469_seamlessbgtest.c4d.zip .