Non standard disk brake post mount I like my Vanmoof B5, but one thing I find very poor is the braking performance. The bike comes stock with 140mm rotors and no-brand mechanical brakes.

I thought I'd upgrade the brakes, going quite a bit overkill for a commuter bike, purchasing a Shimano 180mm rotor for the front and Shimano SLX hydraulic brakes. With this kit, I also purchased a SM-MAF180 disc brake adapter.

How ever, when I mount the rotor and the adapter, the rotor still appears to be approx. 20 mm "too big".

Below is an image of the stock setup. 



Does this fork have a non standard post mount, or have I just purchased the wrong adapter?
Mountain bike forks will normally direct mount to fit a 160mm rotor. The 180mm adapter you purchased will add 20mm to rotor size - so should (might) work with a 160mm rotor. You would need an adapter to increase 40mm rotor size ir you want to go to a 180mm rotor. 

You could try a 203mm adapter or drop back to a 160mm rotor - either might work but I have not had experience with this setup. 

Looking at the post mount, I would be a little concerned the mounts are up to the forces a 180 rotor could generate and would be checking the specs from the fork manufacturer.