Windows 10 laptop with Hi-DPI (3840x2160) touch screen.
Using Qt6.0.1.

Run the following qml file with qmlscene:

qmlscene.exe --resize-to-root flickable.qml

import QtQuick 2.15

Item {
  width: 600
  height: 600

  Flickable {
    id: flickable
    anchors.fill: parent
    contentWidth: 600
    contentHeight: 800

    Rectangle {
      width: 600
      height: 800
      radius: 20
      border.width: 25
      border.color: '#ddd'
    }
  }

  Text {
    anchors.centerIn: flickable
    text: 'contentY: ' + flickable.contentY.toFixed(2) + '\n'
      + 'atYBeginning: ' + flickable.atYBeginning + '\n'
      + 'atYEnd: ' + flickable.atYEnd
  }
}

Try swiping up and down using TouchPad. The flickable's contentY position is capped with 0.5 and (height - 0.5). The content never overshoots as expected. Works fine if you use Mouse Drag, Mouse Wheel, TouchPad Drag, Touchscreen Drag. Does not work only when using TouchPad flick. See the attached video.

Sometimes Mouse Wheel may get stuck in this situation as well, but I assume it is the TouchPad that puts it into that state.

The same behaviour can be observed in any Flickable: GridView, etc..

The issue did not exist in Qt5.15.1.

The issue still exists in Qt dev branch at commit 91dc70dba9fd1f1.