If we declare 'url' property in QML file '/home/root/main.qml' like this:
...
property url imageFolder: "./Images"
...
And then print it out, in Qt5 we got:
"file:///home/root/Images"
but on Qt6 we got:
"./Images"
This cause problem if we try use 'imageFolder' later in QML file which located in other folder,  for example in file '/home/root/foo/Foo.qml' we write:
....
Image { source: imageFolder + "/my.png" }
....
There we expect path "/home/root/Images/my.png", but in Qt6 got "/home/root/foo/Images/my.png"

In General: Behavior or QVariant(QUrl) changed and cause runtime errors

I suspect type conversion code (qvariant/qmetatype), but not sure for 100% and keep investigation going on