The problem was first discovered in a PB10 app but I verified it happens in PB 2017. I am on Windows 10.
The main window of the app is MDIHelp. Clicking a button opens a response window. Clicking a button on the response window opens another response window.
If I close the second response window by clicking the X button in the upper right corner, the first response window is moved behind the main window and becomes unresponsive.