an Autoprop is not a true feathering prop, but an automatically pitching prop. By design, it will go into the "correct" pitch any time water is flowing past it. If you lock the shaft, the "correct" pitch is a very high, nearly feathered pitch - but still enough to put torque on the shaft (and add some drag).

A feathering prop like a Maxprop, or Variprop, or Autostream has a symmetrical blade (no camber) and will produce no torque once feathered completely. However to get it to feather, the blades must drive the hub around the shaft. If the shaft rotates too freely in neutral, it will not feather. If the shaft is locked, the blades will drive the hub causing the blades to feather. And then you can put it into neutral - MAYBE. It takes only a little disturbance to knock the blades into a small angle, then the shaft will start spinning, and might well come unfeathered again.

However a feathering prop, once feathered, will produce little or no torque on the shaft, so any transmission should not be harmed by leaving it in forward or reverse.

One more detail: if you happen to have a ZF/Hurth gearbox, by design it will overrun in the same direction. E.G., if in forward, the prop can spin faster than the engine wants to drive it. More importantly, if the engine is stopped you can spin the prop in forward, if the transmission is in forward (the converse is true for reverse). So you want to put the transmission in reverse to stop the prop, if going forward. The newer large saildrives from Volvo are actually a Hurth product, and work this way.