There is something new in Unity6 that prevents the UI Images (all Graphics actually) from previewing their real (it just does not animate) state without being marked dirty.
I made this slight adjustment (with some extension methods, but you get the idea) in PreviewUpdate method in DOTweenEditorPreview on my side to make it work
foreach (var graphic in CurrentTweens.GetAllTargets<Graphic>())
EditorUtility.SetDirty(graphic);
But this solution is not very great as it requires the object set dirty ...