So I set up a simple scene, and I'm getting the same results. Car is blurry and backplate and Shadowcatcher are not. You can download the files here:
https://evanalexander.digitalpigeon.com/msg/sI_rcOOuEe2tSgIULjfkoQ/Of6cw2gp1GKhXXRFMJER7g# - Motion Blur test
Also - when I turn on IR in the VFB I get the blur on the car, but when I hit RENDER in the VFB window, no blur. in the IR mode, I have to change the Shutter Speed before the blur shows up.
Ultimately, the goal is to have the car be in focus and the backplate to be motion blurred. I'll deal with the wheels spinning, later, now I'm just trying to get the base set up. I can't be the first person doing car renders with backplates and MB here....?
Any thoughts or help appreciated!