There are other reports of this phenomenon on the site over the years. My old motor did it for some time, so I just used the 'plug seatbelt in' solution whenever it happened. My opinion is that it seems to be the seat sensor sticking, as it generally happened after I left my work bag on the passenger seat while going to work.
As I said, it has been reported before and I suspect that a search will pop up a solution much less drastic than seat replacement.
B-Max Zetec 1.4 Petrol
Mid 2015 build
Pretend Spare Wheel