The round definitely hadn't started to chamber, it was completely to the rear of the magazine.
It is possible that my thumbs forward grip led to the issue. I've never had this issue with any standard pressure ammo, so I'm willing to believe the more powerful ammo combined with my somewhat faster shooting could have caused my thumb to move to the slide release.
I will get back out and fire the rest of that box and see what happens ASAP.
Guns don't kill people. Apes with guns kill people! - Robin Williams