Worth a thought ?.
Do you have anything on the back seat at the time ?.
Like a bag or anything with a bit of weight in it ?.
The seats have a pressure pad in the cushion, so any weight and it will trigger the seat belt warning IF the seat belt is not fastened.
I had one person travelling in the rear of our ZS EV Gen 2 last week.
She had her seat belt on and everything was just fine.
Then out of the blue, one of the rear seat belt warning lights was triggered.
She had placed her hand on the next seat to lean across to pick something off the floor.
By applying pressure to the seat cushion with her hand, the car got the idea the seat was occupied and the seat belt was not latched.
As soon as she removed pressure from the seat, it stopped bonging !.