I know you asked specifically about fluorocarbon knots, but just to offer one more idea: have you tried mono or copolymer? I used to have random breakages with fluorocarbon, from several brands and with several knots. I got sick of losing big fish, so I switched to mono and copolymer lines. That solved the problem for me. IMO, quality mono and copolymer lines are great lines and very underrated.