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

Start with the linked mypy playground reproduction using Python 3.12 and mypy 1.10.1, then compare the revealed types before and after sorted() in main.py. The fix is complete when the sorted result preserves the NamedTuple alternatives so the isinstance branches accept sorted_nt.field_a and sorted_nt.field_b without an attr-defined error.