They can't recover stability once they've turned that far.
The
JBM blllistics site includes a
gyroscopic stability factor calculator. Most experts say gyroscopic stability factor values of 1.4 to 1.7 are best for high accuracy work, while Sierra, specifically, recommends 1.3 to 3.0 as best for hunting accuracy. When I run the 68 grain Hornady with a 9" twist at 2700 fps (guesstimated velocity for your loads) I get a stability factor of just 1.25. That is not normally outright unstable, but it's too low for good accuracy. Why most of your bullets fly OK but some don't is probably individual to the gun's characteristics. The short barrel will produce relatively high muzzle pressure and resulting muzzle blast, and if a bullet comes out imperfectly aligned, that extra blast may push it over the edge.
Under the same atmospheric conditions, the same calculator shows the 69 grain Sierra will actually be much more stable with GS of 1.658, so it will shoot better. It's about the same weight, but it is 0.085" shorter, and that turns out to be a critical difference in the velocity and twist rate you are operating in. I'd expect it to shoot fine and not be so hard to tune to an accuracy sweet spot.