A simpler example shows it has nothing to do with annotations -- it is simply behavior of the typing module.

>>> import typing
>>> typing.Optional[str]
typing.Union[str, NoneType]
>>> typing.Optional[None]
<class 'NoneType'>
>>> 

I don't think there's a bug here, and I am closing this as "not a bug". The problem in the original code is that the annotation references a global name that is shadowed by a local (to the class body) name, and because of the initialization, the latter takes precedence.  (To see for yourself, use the dis module to disassemble the code for Spam and Spaz.)