I think one of them is mistransformed. I am thinking it might be the new one that is incorrect.
The piece seems to fill in the gap between the feet when they join together in vehicle mode. It looks like it rotates down to form the base of the foot on a pin going from the white piece to the black base. There is also what looks like an ankle tilt joint visible in the previous shots.
If you look at the picture Husticon linked to a few weeks ago it is consistant with the official pictures