2017 Ford Transit Custom "Hill Start Assist Not Available" Issue

Does anyone have any ideas why this happens?
It happens sometimes with Hill Start Assist switched off as it is in the video and sometimes brakes on its own at low speeds up hill with it switched on.
It's already booked into a dealer.
UPDATE: it's now been to 3 different dealers at total of 7 times, the fault still occurs.
It's had thousands of pounds worth of parts changed under warranty, still the fault remains.
So far the recommendation is to drive with hill start and traction control switched off (switch it off on each journey) and the issue will never occur, but it still does.
Ford are aware of the fault and have a few hundred cases of it across the UK, they don't know how to fix it but claim it's safe to drive... it's certainly not.
UPDATE 2: Now 4 different dealers at total of 8 times...last garage changed the dials... The fault remains.