False positive for class method without Self annotation

Mypy reports an error for method save1 (without annotation) but no errors for save2 and save3 (with different Self annotations), despite their (almost) identical function bodies:

from copy import deepcopy
from typing import Self

class A:
    states: list[Self]

    def save1(self) -> None:
        self.states.append(deepcopy(self))  # error: Argument 1 to "append" of "list" has incompatible type "A"; expected "Self"  [arg-type]

    def save2(self: Self) -> None:
        self.states.append(deepcopy(self))

    def save3(self) -> Self:
        self.states.append(deepcopy(self))
        return self

This bug might be related to #15529 but is not fixed by #15541.

### Tasks

Start by reproducing the reported diagnostic with the Python example in the issue, comparing save1, save2, and save3. Review related issues #15529 and #15541, then trace the Self-annotation handling that produces the inconsistent result. Done means the equivalent methods receive consistent, correct diagnostics and the regression is covered by a test.