The documentation states that only the hi-order bit is reliable : https://docs.microsoft.com/en-us/windows/desktop/api/winuser/nf-winuser-getasynckeystate

but https://cs.chromium.org/chromium/src/sandbox/win/sandbox_poc/pocdll/spyware.cc?q=GetAsyncKeyState&sq=package:chromium&dr=C

uses 0x1 as a mask (extracting the lo-order bit).