Erroneous arg-type error with Mapping and reversed Mapping

I get an arg-type error in MyPy 1.14.1 on the following code using a Union of inverted collections.abc.Mapping:

from collections.abc import Mapping


def fun(x: Mapping[str, int | None] | Mapping[int | None, str]):
    pass


fun({"a": 1, "b": 2})

fun({"a": 1, "b": 2, "c": None})

fun({1: "a", 2: "b"})

fun({1: "a", 2: "b", None: "c"})

The error is:

mypy_type_error.py:12: error: Argument 1 to "fun" has incompatible type "dict[int, str]"; expected "Mapping[str, int | None] | Mapping[int | None, str]" [arg-type]

That is, MyPy takes issue with the parameters in the third function call. The last call including the None key does not produce an error, though. I'd expect the third call to not show an error, either.

まず、報告されたスニペットを MyPy 1.14.1 で実行し、4 つの呼び出しを比較します。次に、引数型のチェックと union-of-Mapping の処理を追跡して、関連する回帰テストの領域を見つけます。完了条件は、他の例を後退させることなく、3 番目の呼び出しが 4 番目と一貫して受け入れられることです。