error in set unions

In the following file, mypy (version 0.950) reports (correctly) ok function as ok, but raises an error (error: Argument 1 to "union" of "set" has incompatible type "Set[str]"; expected "Set[Optional[str]]") in error function.

from typing import Set
from zoneinfo import available_timezones

x = {"UTC"}


def error() -> Set[str]:
    return available_timezones() - set.union(x)


def ok() -> Set[str]:
    a, b = available_timezones(), set.union(x)
    return a - b

Reproduce the issue with mypy 0.950 using the two functions shown in the report, comparing the direct expression with the intermediate variables. Trace the set-union type-checking path and add a regression test covering both forms; done means the valid error function no longer produces the incompatible-argument diagnostic.