The plastic composite V-ilder is probably allowing the v-belt to bottom out which effectively lengthen the drive belt. Also the drive should be a 1/2 x 88.9" Here I use a Stens 265-186 is either 88-1/2 or 89 inch belt depending which website you are to believe. Be careful of after market belts specs are what you get sometimes is not what the printed spec is.
Not sure how the jd is built since we don't work on them. Mainly because we don't chase parts. Another reason is people go to Lowe's Home Depot and buy them and come to us for parts. When we tell them we can't get parts they get mad at us. But that aside look to see if the clutch has a stop where is could hit the the frame somewhere when released. Some models of Husqvarna have this setup and over time grass builds up and and stops the clutch from coming all the way up and not putting enough tension on the belt.