Wick, I've been buying from the same guy for some years now. I sometimes heated beet red and quenched, sometimes let air cool, sometimes dull red and either air cool or quench. About every combination you can use to anneal and I've probably done it. I've have never experienced this, not even close. First round was to a dull red and air dry, after which I did notice it was kind of a mottled light and dark grey, never saw that before and it was enough for me to take note of. It wouldn't bend at all so I heated to a good orange and quenched. it was hard as rock and had a good sized crack.
Like Dave suggested, My first thought was that it somehow had gotten a contaminant.
Robby