Dot artifact in Qt Quick scene graph

After updating from Qt 5.9.2 to 5.12.2, we noticed dot artifacts on the main application after using text fields. The dot artifacts wouldn't go away on screen refresh (would render on top of what was displayed on screen). Following is a video of a sample app that demonstrates the problem, since the dot is small (see upper left of the test app), make sure the video window is at least 512x512 when viewing):
https://drive.google.com/file/d/1SWLtwaliHp78E420Js8CKz7FOj9_Ioca/view

This is the QML used to create the sample app above:

import QtQuick 2.12
import QtQuick.Controls 2.12

Rectangle {
 color: "grey"
 width: 600
 height: 600
 Dialog {
 id: "testDialog"
 width : 512
 height: 512
 contentItem: Rectangle {
 TextField {}
 TextField {
 y: 50 
 }
 }
 standardButtons: Dialog.Ok
 }
 Button {
 y: 520
 text: "Open Dialog"
 onClicked: \{ testDialog.open(); }
 }
}

The artifact seems to appear under the last position of the text field cursor. We reproduced the issue on different video cards with latest drivers (nVidia, AMD, even VMWare's SVGA virtual graphic driver on virtual machines). We are running on Windows 10 1607 Long Term Support Branch with latest updates. If I change the test app back to using 5.9.2, the problem doesn't occur. Also, updating Windows 10 to 1803 or above will also make the problem go away. But our app needs to remain on 1607 LTSB.