Just my guess(correct me if I were wrong):

THe 2 gimballed rocket LO2/LH2 engines could only deflect at a small angle and with small mass, the core will blast past near the released payload and the flame likely would affect the payload. Of course another strategy is stop and then restart the engines but it sounds tricky; any precedent the main engines during launch switched off and restarted?

Besides the Ariana 5, seems only 1)the Shuttle rockets have similar config; if I remember right, the shuttle tank was released just before reaching LEO speed and the shuttle carried storable nitro fuel inside for the orbital maneuver and 2)the defunct Russ Energia rocket