A normal PT6 which is shut down without first feathering the prop, will feather itself as oil pressure bleeds off, though it is not immediate (I tried it in a Twin Otter once). It's an avoid though, as from idle, the prop spins for some time until the blades have feathered themselves enough to really slow down quickly. During that time, the gearbox is not being supplied pressure oil I was told.
Though I doubt it on this aircraft, some PT6 propellers have blade latches to hold the blades in flat pitch after shutdown - generally for floatplane operation.