There is other related case:

>>> from typing import *
>>> class P(Protocol): pass
... 
>>> class B:
...     def __init__(self):
...         self.test = 'OK'
... 
>>> class D(P, B):
...     pass
... 
>>> D().test
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'D' object has no attribute 'test'