Workaround: I've compiled Qt 6.4.1 with the fix form Qt 6.4.2 and attached the qwindows.dll binary to this bugreport. Just replace the installed version from c:\Qt\qtcreator-9.0.0\bin\plugins\platforms\qwindows.dll and this problem goes away!

Second workaround: Another solution is to use a fonts.css and start Qt Creator as: qtcreator -stylesheet fonts.css The following fonts.css made the UI font size normal again:

QWidget
{
    font: 9pt;
}