On Ubuntu 24.04, GNOME 46, with QPA platform "xcb", the following QML code fails to open a maximized window:

import QtQuick 2.15
import QtQuick.Controls 2.15
import QtQuick.Window 2.15

ApplicationWindow {
    id: root
    visible: true
    visibility: Window.Maximized
    Text {
        text: "Hello world"
    }
}

See attached screencast. The window is not maximized but once the window frame is manipulated, it "snaps" into the maximized state.

I've found a workaround which is to set it to AutomaticVisibility in QML and then once the component is created, I set it to Maximized from C++.