I figured out there are more details probably worth considering:
- there is a special behaviour when GetOverlappedResultEx is called without event in overlapped but with the last bit of file handle set to 1 (it behaves like GetOverlappedResult in this case it seems); - there supposed to be dependency of GetOverlappedResult() behaviour (as Ex or as it works now) based on app manifest (https://learn.microsoft.com/en-us/windows/win32/w8cookbook/application--exec...).
So maybe accounting for that will need some code reshuffling, I will resend once figure out all the details.