It happened to me yesterday with double uni knot. I normally use either 4lb copolymer Izorline or 6lb FC red label. Yesterday tried to connect FC to 6lb nanofil and 2 broke while pulling tight. I switch to 4lb Izorline and good to go. I think I was in such a hurry and didn’t clinch it slow enough. The heat from sharp braid can kill FC line with ease. I don’t have problem when do it slowly, 5 wraps on each with a lot of wet before pulling it tight.