The actions "Lower selection" and "Raise selection" are not considered in the undo/redo list.
This behavior is not consistent with the UI of almost every other diagram/image editor (Dia, Inkscape, OpenOffice Impress, etc). It's often confusing to understand what happens when an undo "skips" the z-ordering and undoes the previous action.
To reproduce it: create a new diagram, add two superposing objects, then select one of them, perform Raise/Lower selection and then Undo. It will undo the "added object" command, not the "Raise/Lower selection".