RSMB Pro Vectors Incorrect Results


Hi all. Hoping to get some help with RSMB Pro Vectors.

The problem seems to be that RSMB is totally ignoring my motion vector layer, and blurring the entire scene.
I Have a simple test scene with some cars, a background plane and some pillars. I want only the cars to be blurred, but the pillars are also affected completely.

Here is a snapshot of the scene with RSMB applied, as well as my MV layer. The MV Layer is transparent aside from the subject (cars) that I want to blur

As you can see, the background pillars are blurred where they should be unaffected by the MV. Additionally, the blur levels for the pillars will alternate between fully blurred & no blur as the cars pass them in the animation, so it really does seem like RSMB is ignoring my MV pass.

I am using 32bit EXRs
Cryptomatte to mask the subjects in the MV pass
MV pass rendered using C4D R21
My composition contains RSMB applied to an adjustment layer, with RGB underneath, and a precomp with cryptomatte (mask) & the MV pass.
MV pass & cryptomatte set to preserve RGB (for linear preservation) although no different if this is turned on or off.
My RSMB Vector settings are:
Blur amount 0.5
Max displacement 64
Vec scale x,y 1 (default)

I have tried adjusting these settings, as well as adjusting the ExtractoR settings for the MV pass between -1,1 and 0-1 with no changes.

If anyone has had difficulties with RSMB before and found a solution, please help! I’m going crazy.