I MAKE items from Nylon and Polypropylene and have NEVER had a problem with
it stretching. I suppose there are different grades of nylon, But nylon is
used because it DOESN'T stretch!!!
I have used nylon latigos with no problem. Perhaps your saddle does not fit
properly or it was noth cinched up well. Often the cinch needs to be
retightened after about 10-15 minutes. Which is why we girth up a horse,
walk him around THEN retighten.
Godd luck on your repairs.
Teddy