QScreen::devicePixelRatio returns wrong value on macOS system with multiple displays and scale factors

I have a macbook pro w/retina display connected to an external non-retina display. If I launch my application on the retina display and run the following code:

 auto screenNumber = qApp->desktop()->screenNumber(this);
 auto scaleFactor = qApp->desktop()->screen(screenNumber)->devicePixelRatio();

I get 0 for the screen number, and 2 for the scale factor as expected. If I drag the window over to the non-retina screen and run the code again, I get 1 for the screen number but still 2 for the scale factor which is incorrect. If i launch the application on the non-retina screen initially I get the opposite problem and the scale factor is always 1.