sorted() loses information about namedtuple elements

Bug Report

it seems that running sorted() on an union of iterables of named tuple, create some missing information

To Reproduce

playground url

from typing import NamedTuple
from typing import reveal_type


class NamedTupleA(NamedTuple):
    field_a: str


class NamedTupleB(NamedTuple):
    field_b: str


def my_fun(my_list: list[NamedTupleA] | list[NamedTupleB]) -> None:
    reveal_type(my_list)
    # Revealed type is "Union[builtins.list[tuple[builtins.str, fallback=__main__.NamedTupleA]], builtins.list[tuple[builtins.str, fallback=__main__.NamedTupleB]]]"

    for nt in my_list:
        if isinstance(nt, NamedTupleA):
            print(nt.field_a)
        else:
            # here nt is recognized of `NamedTupleB` type, I can access field_b
            print(nt.field_b)

    reveal_type(sorted(my_list))
    # Revealed type is "builtins.list[tuple[builtins.str]]"

    for sorted_nt in sorted(my_list):
        if isinstance(sorted_nt, NamedTupleA):
            print(sorted_nt.field_a)
        else:
            # here I expected nt to be recognized of `NamedTupleB` type
            print(sorted_nt.field_b)

if __name__ == '__main__':
    my_fun([NamedTupleA('value_1')])
    my_fun([NamedTupleB('value_2')])
    # this error is expected: I cannot mix NamedTupleA and NamedTupleB
    my_fun([NamedTupleA('value_1'), NamedTupleB('value_2')])

basically I have 2 list of NamedTuples.
my_fun must accept either one.
Ideally I could write a User-Defined Type Guards, but let's say that I'm just checking (with isinstance) each element type.

If I loop directly over the list, the check works for both the if and the else branch (there shouldn't be any other case, the union is between two types)

If I loop directly over the sorted list, the type of the element is different, so in the else branch mypy gives me an error

Expected Behavior

I expect no errors on print(sorted_nt.field_b)

Actual Behavior

main.py:32: error: "tuple[str]" has no attribute "field_b" [attr-defined]

Your Environment

linux, python3.12, mypy 1.10.1

Python 3.12 と mypy 1.10.1 を使用して、リンクされた mypy playground の再現例から始め、main.py の sorted() の前後で表示される型を比較します。sorted() の結果が NamedTuple の各候補を保持し、isinstance 分岐で sorted_nt.field_a と sorted_nt.field_b が attr-defined エラーなしに受け入れられるようになれば、修正は完了です。