UI Artifacts #256

I tried QWindowKit version 1.5.0 with several Qt versions:
5.15.2
6.5.3
6.8.3
6.10.3
6.11.3
6.12.0

They all cause UI artifacts (lines or streaks or weird button see-through) when double clicking the bar to maximize or normalize the window.
This same effect also happens when using Windows (11, in my case) snap features.

For Qt6, these artifacts also happen when you resize the window.

See attached screenshot for the artifacts. Black and white lines appear. Note, I blurred the left side of the UI.

When using the top-right maximize-button, this effect is not present.
I connected the following action to that button:
QObject::connect(btnMaximizeApp, &QToolButton::clicked, this, [this] () {
if (isMaximized()) {
showNormal();
}
else {
showMaximized();
}
});

Is the default doubleclick behavior different from Qt? I cannot overrule it, because the click-event is handle by QWindowKit before Qt even sees it.