Undoing UDocument Sort Order does not take effect, e.g. the value is changed in the Inspector but the hierarchy isn't updated and no change happens in the game view.

See repro scene where two root UIDocument component (with the same panel settings) start with Order = 0.
Steps:

1) Import the provided .unitypackage
2) Open the UIDocumentSort scene
3) Enter playmode
4) Observe "UIDocument Blue" appears on top of "UI Document Red"
5) Select "UIDocument Red" and change its priority to "1"
6) Observe "UIDocument Red" now appears on top of "UI Document Blue"
7) Undo this change and see the value revert to zero in the inspector

Expected result: "UIDocument Blue" appears on top of "UI Document Red" again
Actual result: UIDocument Red" still appears on top of "UI Document Blue"

Reproduces with trunk (2021.2)