Fenders will rattle when you come across uneven road surface.
Bad design. Rear fender lacks additional points of support. Tightening it can temporarily fix the rattling issue, but the fender movement will either loosen the screw or (eventually) brake the holder.
Create additional points of support to increase fenders stability. These are different solutions from different Mate X owners.
Here is one of those solutions with measurements and specifications made by one of the Mate owners posted in one of the FB groups (link)
A similar solution, a bit more elegant because it’s less visible. (link)
Find generic fender holders or create your own
Use the back reflector holder to stabilise the fender. All bikes were shipped with the front/back reflectors and their holders.