Use the following command to create a screenshot of a simple Qt6 app on Windows (rendering with D3D by default): 
ffmpeg.exe -f gdigrab -framerate 30 -i title="QML Window" -vframes 1 output.png 

you can find the it will not work.

Attached qml app:

import QtQuick
import QtQuick.Controls

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    Text

}