I kept thinking “why doesn’t the ball act like a knuckleball pitch?”.
I had to look up why a knuckleball flight is unpredictable. Previously it was thought it was do to airflow over the seams. The study I read about proposed that it was due to “drag crises”.
In drag crisis, the flow of a thin layer of air that surrounds the ball flips between turbulent and smooth. This abruptly changes the drag on the ball.
This is dependent on the speed of the ball which is probably why knuckleballers pitch at a much slower speed.
In this case the ball is supersonic for its entire flight path. Supersonic flow is very different from subsonic, since a shock wave is formed and the surface of the object experiences flow in the wake of the shock wave. This may reduce turbulence and stabilise the flight of the ball. I do know that turbulence and drag increases dramatically in the transonic regime (where parts of the object are experiencing supersonic flow and others subsonic speeds), then falls again once the entire object is supersonic. This may be consistent with less turbulent flow when supersonic. (This is why the Concorde only needed its afterburners for takeoff and accelerating through Mach 1. Once supersonic, the regular thrust from the engines was sufficient to accelerate to Mach 2 and cruise.)