That indicates that the belt is a little too long and the pulleys are out of alignment .
Once a belt has flipped then it is trash as it damages the fiber reinforcement .
Try babying a 68 in a plain belt ( to save money ) for a while to see if it flips as well.
If not then get a proper kevlar one as plain belts do not last long .
When faced with your situation I originally used a length of rope ( 1/2' or 5/8" ) to replace the belt then pulled it tight enough to stretch the tension spring 1/3 to 1/2 of the travel of the tension arm
The rope was then removed and the length measured which gave me a good starting point .
From there I picked the cheapest belt near that size to see how it ran.
I do the same thing now except I have a 244" broken A belt to do the measuring with .