Cannot annotate child of a `Generic[AnyStr]` with an `Optional[AnyStr]` attribute

Consider the following code:

from typing import AnyStr, Generic, Optional

class Base(Generic[AnyStr]):
    def __init__(self, data: AnyStr) -> None:
        self.data: AnyStr = data

class Child(Base):
    def __init__(self, data: AnyStr) -> None:
        super().__init__(data)
        self.buffer: Optional[AnyStr] = None

    def act(self, s: AnyStr) -> None:
        self.buffer = s

Running mypy on this code produces the following errors:

anystr01.py:13: error: Incompatible types in assignment (expression has type
"str", variable has type "Optional[AnyStr]")
            self.buffer = s
                          ^
anystr01.py:13: error: Incompatible types in assignment (expression has type
"bytes", variable has type "Optional[AnyStr]")
            self.buffer = s
                          ^
Found 2 errors in 1 file (checked 1 source file)

If I try to address the problem by changing the child class declaration to class Child(Base[AnyStr]):, mypy instead emits the following:

anystr01.py:9: error: Argument 1 to "__init__" of "Base" has incompatible type
"str"; expected "AnyStr"
            super().__init__(data)
                             ^
anystr01.py:9: error: Argument 1 to "__init__" of "Base" has incompatible type
"bytes"; expected "AnyStr"
            super().__init__(data)
                             ^
Found 2 errors in 1 file (checked 1 source file)

Is this a bug, or am I simply doing something wrong?

Your Environment

• Mypy version used: both 0.961 and commit 132b8e527730b654a3f3d04684bebc9daff6cabd
• Mypy command-line flags: --pretty
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.9.13
• Operating system and version: macOS 11.6.6

Use the supplied Python snippet as the reproduction case and run mypy with the reported flags. Trace generic inference and assignment checking for Child, Base[AnyStr], and Optional[AnyStr]; done means the behavior is corrected or clearly documented, with regression coverage for the case.