Looks like this behavior is intentional.  This subclass adds a '%(default)s' string to the help lines if:

    if '%(default)' not in action.help:
        if action.default is not SUPPRESS:
            defaulting_nargs = [OPTIONAL, ZERO_OR_MORE]
            if action.option_strings or action.nargs in defaulting_nargs:
                help += ' (default: %(default)s)'

So it adds the default if it is an `optional`, or a `positional` with '*' or '?'.  

A `default` value for a required positional (including '+') does not make sense, since the user input always overwrites the default.  

(I recommend closing this issue).