Are you obliged to stop in this situation?

Why?

The officer has his back to you, which means you must stop and must not enter the junction. The green light does not change this because the officer’s signals take priority over traffic lights.

art. 5 ust. 2 Prawa o ruchu drogowym; § 108 ust. 3 rozporządzenia w sprawie znaków i sygnałów drogowych