Yes I've been working with a QT Quick app. As you say, the keyboard doesn't appear normally, but I forced it to be visible. In my App.qml file I have modified the auto-generated InputPanel:
property bool showKeyboard : active
//y: showKeyboard ? parent.height - height : parent.height
y: parent.height - height
Not that this fixes the keyboard. It still can't be used to enter text. But when I press a key I see an error "input method is not set" in the browser console. I'm not familiar with the input system, but maybe that's useful info for your debugging,
I will try to see if I can get the keyboard working with a QML widget in a widget app. Maybe that will be a suitable workaround.