Evenin,
First off let me just say that this is awesome gui system :-)
I think I may have noticed a bug in the 3.0.7 RC2.
I have a sliced sprite which I am using as my background, I set the sprites anchors to be Padded and then dragged in the 'UI Root (2D)' and everything appears to work fine (stretch wise). I then save the prefab (by dragging and dropping it back in to the project view) and remove it.
Now comes the bug, when I add that prefab back to my MainPanel the anchors on the sliced sprite that I intended to use as a background have disappeared.
Cheers for your assistance and keep up the good work.