The background of your composition does not render. That's just there for your previewing purposes. If you want the background to actually be white when you render, create a white solid and put it under everything else in your layer stack.
I agree with Szalam---partially. The problem is that this behavior through AME is different than when rendering and exporting directly from After Effects. I'd call this difference a bug.
Please submit a bug report here:
Thank you! This surely fixed the problem!