Incorrect type narrowing when unpacking union of tuple on an existing variable

Bug Report

When unpacking a tuple with Union types over existing variables, some of the types disappear.

To Reproduce

import random


def foo() -> tuple[int, None] | tuple[int, str]:
    if random.choice((True, False)):
        return 0, None
    return 5, "thing"


res = None

status, res = foo()
reveal_type(res)  # builtins.str

Expected Behavior

res should be an Optional[str], not a str.

Actual Behavior

The Optional gets lost in the assignment.

Your Environment

• Mypy version used: 0.950
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.10
• Operating system and version: Manjaro Linux x86_64 5.15.41-1-MANJARO

This may be related to #9731, but I'm not sure about it.

The issue names no repository file or test; start by running the provided reproducer with mypy 0.950 on Python 3.10 and review related issue #9731. Trace tuple-unpacking assignment and type narrowing, with completion shown when reveal_type(res) reports Optional[str] rather than str.