Polyester is tough sometimes
I have an 18g soft copolyester, It is very tough to tighten the clamps so the string wont slip and also avoid any crushing of the string at least with neos clamps. Seem to do much better with Babolat Star 5 clamps. Although if you dont tighten clamps enough with Star 5 you will shred the poly. If the poly slips through the neos clamp there will most likely be no damage to string in my experience.