Maybe it's my long arms, but I've had 4 bows made by three different bowers break in the last 5 years. All while using FF string.
I've broken quite a few over the past 20 years or so. I'm pretty sure all of them had a dacron string on them--like 99.99% sure. I don't blame the string though. It's funny--Jack Harrison's warranty states that if you use a dacron string on one of his bows, you void the warranty. Go figure.
Art, one of my bows is osage (the longer one), the other is hickory. Both are 60-65# at my draw length (30-30.5"), and I shoot POC's from both with 125 grain points (not real heavy, not real light-9-10 gpp). I use a 10 strand Dynaflight '97 string (2-bundle) with the loops padded to 18 strands with dacron. The osage has narrow, deeper cored limbs while the hickory has wider, thinner limbs.
Just to clear the air, I'm not doubting your results at all, and don't blame you for suspecting the strings. I wish I knew of a way to figure it out for sure.