Fitness app pace graph inaccurate for treadmill workouts

I can sync my Apple Watch Ultra and my iPhone 17 Pro Max with a Star Trac treadmill. The watch and phone display exactly the same time, pace, and heart rate as displayed on the treadmill. (I wear a Garmin heart rate strap that connects to my watch and phone and apparently that data is transmitted to the treadmill.) After the run the distance and average pace are correct in the Fitness app HOWEVER, the pace graph is ridiculously wrong with paces of 1.4 min/mile varied with other crazy paces and a few that are correct. How can I fix this problem? It would be great if the Fitness app would capture and report all of the pace data sent by the treadmill.