I have the same problem. Sometimes after write PIN/password and press login, pressing Ctrl+Alt+Delete should do the trick. Sometimes you need to scroll up the cover, but other times not. Or a combination of both.
This is one of the trillions unresolved bugs Windows 10 has, going version across version and makes me having my SP4 stored in a drawer. Never Windows has been so buggy and had so much time unresolved problems.