Tires. Ask them to put a set of new or demo tires on the car …maybe front first then rear. Eventually they’ll zone in on which one is faulty.
Happened to my 2018 MS 100D. After 4 visits and zero issues reported from an engineering perspective, it came down to one tire where that foam was loose and it was making a difference at higher speeds. Passenger rear if I recall. They changed the one tire and it was like a light shone on the car all of sudden after 2 months of headache.
You have to convince them to actually take the time to swap a set from a demo model. They’ll never use brand new tires as they won’t devalue them.
Goodluck! If not, sell it open market. You’ll come out ahead