What are you trying to do by interrogating the OSK? It seems to run as a higher user than normal, so that is likely why you cannot interrogate it. You might run Studio as an admin and that might allow you to interrogate it. You'd also have to have your users launch Runtime as an admin to work with it, so that is likely not a workable scenario.
We have a windows application containing two text boxes for username and password. Copy-paste functionality is disabled. We have tried both sendkeys and set text methods. But both are failed. So we tried on-screen keyboard method. For that, I couldn't interrogate the osk file. Only can interrogate using Monitor all start method. But while using that I couldn't get two methods after stopping the interrogation. It's fine when using the test target method.