Part of the issue 19030 patch was incorrect and we missed it in the review. Specifically, the else clause in this bit:

            try:
                get_obj = getattr(cls, name)
            except Exception as exc:
                pass
            else:
                homecls = getattr(get_obj, "__class__")
                homecls = getattr(get_obj, "__objclass__", homecls)
                if homecls not in possible_bases:
                    # if the resulting object does not live somewhere in the
                    # mro, drop it and go with the dict_obj version only
                    homecls = None
                    get_obj = sentinel

The restriction that the __class__ of the object returned by a descriptor must appear in the MRO doesn't make any sense. The entire else block should be simplified to just:

    homecls = getattr(get_obj, "__objclass__", None)

And a "defining class" of None should be documented as a possible result for dynamic attributes that appear in __dir__ but don't set __objclass__ appropriately, and don't correspond to a descriptor.