I doubt it's bad spool. That's just so unlikely. These companies, particularly the Japanese ones have great Quality Control.
I won't hazard a guess of what happened to you. But what it looks like happened to you happened twice to me last season with Seaguar Basix and it was my fault not the line's fault. I have been using mono my life when not fly fishing and I didn't take into account that fluoro has to be taken care of better.
I was making long casts and every now and then I'd get a very mild backlash without a tangle. So instead of picking it out I let it ride thinking I could get it out on its own on the next cast. In my opinion I just set myself up for failure because sooner or later that brittle fluouro carbon will get knicked.
So I'd make a booming long cast and then suddenly half my line was gone because that knick became a complete fracture. Lost a nice Megabass Spinner Bait and a Swimbait doing this; breaking off in mid flight.
Now I take real good care of backlashes, and I use backing on the spool so that I only have 60-70 yards of fluoro spooled up and I am wasting less lane. That way I can spool three times with a 200 yard spool. Once you get that knick in your line, it's a matter of when not if.
What do you think of the uni knot?