Not sure but a couple of things come to mind

I have had the same problem with old K&B engines with the removable shaft. If that is your case obtain a hardened stud from an auto supply and increase the torque.

I have seen problems caused by the starting method. Many will "spin up" the starter before engaging the spinner to compensate for a too weak starter/battery or by habit that will act as an impact driver eventually loosening the prop.

Last but not least the torque of the nut may not be enough. I know you think it is being tightened sufficiently but those little boogers have to be "oilfield tight"(as tight as you can get it and then one more round)LOL just kidding but really, increase the torque a little bit and see what happens.