QWin32PrintEngine incorrectly unsets custom size in some cases

When using a Zebra Label Printer (ZDesigner GK420t), when the printer property "Override Application Default Settings" is unchecked, launching a QPrintDialog will reset the page size to 0.75 x 0.25 inches if accepted.

Tracked it down to QWin32PrintEnginePrivate::setPageSize, in qprintengine_win.cpp lines 1605 and 1612 (in 5.6.1):

1605:
const QPageSize printerPageSize = m_printDevice.supportedPageSize(pageSize);
Starting at 1612:

if (printerPageSize.isValid()) {
    has_custom_paper_size = false;
    devMode->dmPaperSize = m_pageLayout.pageSize().windowsId();
    devMode->dmFields &= ~(DM_PAPERLENGTH | DM_PAPERWIDTH);
    devMode->dmPaperWidth = 0;
    devMode->dmPaperLength = 0;
}

The pageSize is "supported" as it is the default size set in the driver, so printerPageSize is valid.  However it's still technically a custom size (windowsId() returns 256), so dmPaperSize is set to 256 while dmPaperWidth and Length are both set to 0.  PrintDlgEx doesn't like this, and chooses 0.75 x 0.25 to fill in the GUI, thus silently changing the page size when the dialog is accepted.

I haven't been able to fix this in code without complicated checks or outright overriding the size returned by the dialog, and we serve a large a portion of customers with this brand of label printer