I have a SA5-271 running Windows 10 build 1903. Even when purchased new, with build 1701, I had the same issue. I use PIN to log in, and when the detachable keyboard is removed, the on-screen number pad does not appear to allow me to enter my PIN, I have to use the on-screen keyboard from the accessibility menu. However, if the detachable keyboard is attached, the numpad does show up.
I have tried scouring all the settings possible, but nothing seems to fix this issue. Does anyone else know of this issue and a work-around for it?