Reproduced on 5.2, 5.3;

Repro steps:
1. open attached project "uiInput.zip";
2. build to Universal 10 and deploy on Surface 3;
3. Switch to "tablet mode". Connect cover keyboard, type something in the input field, then disconnect cover keyboard. Try selecting input field. Virtual keyboard should appear, but it doesn't. No hardware keyboard connected;

About tablet mode: http://windows.microsoft.com/en-us/windows-10/getstarted-like-a-tablet