We narrow in the first case below but not the second (playground):

def maybe_int() -> int | None:
    return 1

def _():
    x = maybe_int()
    if x is not None:
        reveal_type(x)

    if (y := maybe_int()) is not None:
        reveal_type(y)

Anywhere we would narrow on a test against a Name, we should narrow in the same way if that name is the target of an assignment expression in the same test.