mypy can not correctly tell when optional type can not be None

Bug Report

mypy can not correctly tell when optional type can not be None if the variable was previously set or checked in @property.

To Reproduce

class Test(object):
    def __init__(self) -> None:
        self.test: str | None = None

    @property
    def is_set(self) -> bool:
        return self.test is not None

    def failed_check(self) -> None:
        if not self.is_set:
            return

        # self.test can not be None
        value = self.test.strip(',')
        print(value)

class Test2(object):
    def __init__(self) -> None:
        self.test: str | None = None

    def set_me(self) -> None:
        self.test = 'test'

    def use_me(self) -> None:
        self.set_me()

        # self.test can not be None
        self.test.strip(',')

Expected Behavior

No issues found.

Actual Behavior

test.py:14: error: Item "None" of "Optional[str]" has no attribute "strip"  [union-attr]
test.py:28: error: Item "None" of "Optional[str]" has no attribute "strip"  [union-attr]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 0.991 (compiled: yes)
• Mypy command-line flags: mypy test.py
• Mypy configuration options fro m mypy.ini (and other config files):
• Python version used: Python 3.10.8

Start by saving the examples as test.py and running mypy test.py with the reported Python and mypy versions. Trace how mypy narrows Optional values after property checks and method calls; done means both examples produce no errors, with regression coverage for the reported behavior.