I assume you've checked the tyre sizes are correct and tread depths similar...
It's an issue that RAC patrols had on the first ABS equipped Transits (About 2004~). When they pull up at a roadside breakdown, just before stopping the wheels are turned to full lock. This used to bring on the ABS, and when on loose gravel the vans wouldn't stop.....
I think you may find, this has been a fault on the van since new, and the driver has never noticed!