periodic hard shift from 2nd to third or form third to send

What is your question?

hear and feel a large thump that shakes the car to all passengers as the transmission seems to drag at low rpm to 1500, traveling at 15 to 22 miles per hour, and then abruptly shift up to 3rd or from 3rd down to 2nd.. It has happened on level ground , slight inclines, moderate inclines, turning left while accelerating from a stop. current mileage 42800 Not aware of any trouble indicator codes being displayed. Dealer has test driven twice without replicating the problem

What makes this problem better or worse?

not predictable

How long has your 2016 Buick Enclave had this problem?

intermittently for the last two years