MDI issues on Windows with mixed-resolution displays

I have a high-resolution built-in display and a standard-resolution external monitor. Running on Windows 10 with Qt 5.9.5, I see issues with the MDI example program (qt\qtbase\examples\widgets\mainwindows\mdi) which also affect our Qt-based application. (I am currently not in a position to test with Qt 5.10.)

Here is my display configuration as set in the Windows 10 Display control panel:

Display 1
Built-in high-resolution laptop display; resolution set to recommended 3840x2180; scale and layout set to recommended 250%.

Display 2
External standard-resolution monitor; resolution set to recommended 1920x1200; scale and layout set to recommended 100%.

Issues with standard-resolution monitor as my main display

Display 2, the standard-resolution monitor, is set as my main display. 

I compiled the MDI example in Qt Creator but I am running it from the desktop.

Run mdi.exe and position it on the high-resolution display.

Choose File->New. A new window appears. It is roughly 1/2 the expected width and height. The window's title bar is roughly 1/2 the expected height. The text in the title bar is expected size and so is cut off. The icons in the title bar are roughly 1/2 the expected size.

If I drag the MDI frame to the standard-resolution monitor, everything appears correct.

Issues with high-resolution monitor as my main display

If I change my main display to the built-in high-resolution laptop display and reboot, and run mdi.exe on the high-resolution laptop display, I see the following:

When I create new document (File->New), the window is tiny, though the title bar and title bar icons are correct.

If I maximize the document window, the document minimize, maximize and restore icons are miniscule.

If I minimize the document window, the minimized window shows no title, just the maximize and close icons.