Flight Instructor with student reported during training flight they attempted to shut down the left engine but it began to windmill resulting in the Flight Instructor landing at nearest suitable airport.

Narrative

During engine fire in flight simulation; student correctly ran through immediate action items; brought left power lever to idle and turned engine master OFF on LEFT engine. Approximately 110KIAS; engine feathered and shut down but continued to windmill consistently at approximately 700 RPM (IP (Flight Instructor) estimated by sound of engine). Reduced airspeed; propellor still windmilling. IP asked SP (Student Pilot) to consult AFM emergency section for anything to do with windmilling propellor; was unsure of best procedure to follow. Airworthiness Directive 2018-18-02 states'...allow windmilling restart only as an emergency procedure.'; which goes against AFM. Since we were only approximately 10 NM southeast of ZZZ at 7500' MSL we squawked XXXX; [requested priority handling] with ZZZ CTAF (XXX.X) and joined the right downwind Runway X for a one engine inoperative landing. Eventually during descent; left engine stopped windmilling with feathered propellor. Precautionary landing felt like the best option given the knowledge of the AD and proximity to suitable airfield. IP did not want to risk damaging engine by attempting windmilling restart. Airplane at ZZZ; feathered left engine on ground.