The booster has the wrong moment of inertia when detached from the upperstage.

This rocket has an automated reusable booster. The upperstage detaches the booster and then switches to it. The booster will then flip around and boost back. After boosting back, the booster switches back to the upperstage. The upperstage performs circularization until the booster hits 30,000 m in altitude, then it switches back to the booster, which performs the landing.

However, when I don't allow the booster to switch back to the upperstage and follow it until it lands, the landing glide goes horribly wrong and the rocket is unable to land accurately or at all. It appears that the rocket meneuvers much more slowly that it should, as if it has a larger moment of inertia than it should.

To reproduce the issue, disable AG 4 DURING the boost back burn. This should stop the booster from switching to the upperstage, therefore preventing the booster to be reloaded when the upperstage switches back to it (which updates its moment of inertia).