This bug interests mainly (or only) Windows with a non-integer scale factor set (tested with 125%).
Since Qt 6 I am using a Floor or Round HighDpiScaleFactorRoundingPolicy to avoid the known painting errors caused when such a scaling factor is set (QTBUG-90975).
Now I have realized that this appears to break the full screen behaviour.
If I call QWidget::showFullScreen() when the rounding policy is, for instance, Floor, the widget will not fill the whole screen but only a portion of it. This doesn't happen with Qt 5, where it will fill the whole screen.
Leaving the policy to the default value, means I get the glitches but full screen woks as expected.