PyCharm PyTypeChecker complaining about type signature when updating set with set comprehension

Here is a minimal example:

from typing import Set


def f() -> Set[str]:
    xs = set()
    xs.update({x for x in ["a", "b"]})
    return xs

PyCharm's PyTypeChecker will complain about the function's type signature:

Expected type 'Set[str]', got 'Set[set]' instead 

If I replace the set comprehension {x for x in ["a", "b"]} by set value {"a", "b"}, there is no warning.

Why is that so ?

I'm using Python 3.8 and PyCharm 2021.2 Community Edition.