My GT has 9,000 miles and I usually drove it in Normal mode. I also noticed this jerk changing to 5th gear, but not that sharp as changing to 3rd. These days I am driving in S mode, as in Normal 2nd and 4th gears are skipped usually, maybe that causing that sharp jerk when changing from 2-3 or...
I have a 2019 Mustang GT with A10 gearbox, 3rd gear is changed with a sharp jerk. It is noticable especially when I switch gears manually using the flips from 2-3 or 4-3. From driving point of view it is a good feeling (my head is shaking :-)), but I am worrying as other gears change smoothly...