As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE INSTRUCTOR PILOT PERFORMED EMERGENCY PROCEDURE TRAINING, SHUT DOWN AND SECURED THE LEFT ENGINE. UPON ATTEMPTING TO RESTART THE ENGINE, THE LEFT PROPELLER WOULD NOT UN-FEATHER. LANDED WITHOUT INCIDENT. MAINTENANCE CREWS DISCOVERED THE LEFT PROPELLER UNFEATHERING ACCUMULATOR WAS DEFECTIVE AND HAD IT OVERHAULED. OIL WAS FOUND IN THE ACCUMULATOR.