100-400L 5D2 viewfinder curiosity, FF owners chime in

I've owned a 100-400 before but only used it on crop, I recently acquired another one which I'd use on my 5D2. I just noticed something strange in the 5D2 viewfinder with the 100-400mm mounted. There is a clearly defined circle at 400mm in the viewfinder, the circle doesn't show up at 100mm

The funny thing is this circle doesn't show up at all on my 5Dc even if I stop down. On the 5D2 it becomes more pronounced as I stop down and hold the DOF button. Could it be something with the focusing screen? The circle does not show up in any pictures. I have the regular AF screen in both cameras.

5Dc 400mm

Correction, the circle does show up at 100mm as well on the 5D2 when the lens is stopped down. Has to be the focusing screen right? since the shape stays exactly the same. Nothing on the 5Dc though even fully stopped down.