The problem is basically that _parse_known_args calls _parse_optional to determine whether something is an optional or a positional. But _parse_optional only checks "if arg_string in self._option_string_actions", that is, if the string can be found in the main parser actions.

So either this method needs to check the subparser actions, or the "if arg_string in self._option_string_actions" stuff needs to be delayed until the subparser. Neither of these seems like a simple change, but I agree it's a bug.