@jef-n While fixing this issue, I moved the whole QCoreApplication::addLibraryPath block (including bit for Windows) to before the splash screen setup. Is this cool for Windows builds?

It seems that the splash setup causes the Qt plugins to be loaded. So, to me, it makes sense that regardless of platform, QApplication should have any custom Qt plugins path added before then. Certainly fixes a longstanding issue we had on Mac that I could not quite figure out before.