The behavior in this example was changed in #18538:

def any():
    return 1

def f() -> None:
    x = "a"
    x = any()
    assert isinstance(x, int)
    reveal_type(x)  # "Never", but used to be unreachable (no message)

In the past, the code after isinstance was silently unchecked. Now the inferred type is Never, which will likely generate false positives. Neither behavior is great, but users may consider the switch from false negatives to false positives to be a regression. I'm not sure what we should do here. If we'd allow variables to be redefined freely, there would be no issue.

cc @ilevkivskyi who authored the PR that changed the behavior