Error when using TypeVar in type annotation for @property

from typing import TypeVar, Generic
from pyspark.sql import DataFrame
import pandas as pd
AnyDataFrame = TypeVar('AnyDataFrame', pd.DataFrame, DataFrame)

class OutputA(Generic[AnyDataFrame]):

    @property
    def original_data(self) -> AnyDataFrame:
        return self._original_data

    @original_data.setter
    def original_data(self, value: AnyDataFrame) -> None:
        self._original_data = value

I get mypy error: Need type annotation for "_original_data" [var-annotated]

I have to add the annotation as below to solve the error, why?

    @original_data.setter
    def original_data(self, value: AnyDataFrame) -> None:
        self._original_data: AnyDataFrame = value

Reproduce the reported Python snippet with mypy and inspect how the property getter, setter, TypeVar, and Generic class are analyzed. Read the diagnostic path for the unannotated _original_data attribute; done when the behavior is explained or the issue is narrowed to a reproducible type-checking defect.