With "Show only when no keyboard attached" option OSK button is always visible - with keyboard attached and also unattached, on Windows version 22631.3447, app 3.7.9.
When I turn off the "enhanced classic taskbar" and in windows settings set OSK button to same option it works properly - so it's probably bug in the app (changing option in windows settings with the enhanced taskbar on has no impact, keyboard button is always visible).