To reproduce:

1. Download attached project "BugReport_UI_Canvas_Odd_Behavior.zip" and open in Unity
2. Open "Scene" scene
3. Move "Canvas 2" into "Canvas 1" in Hierarchy window
4. Select "Canvas 2" and disable or remove the "CanvasScaler" Component
5. Observe how this affects the size of the green image, which is part of "Canvas 1"
6. Click Edit > Undo or Ctrl + Z couple times until it undoes your changes
7. Observe it undoes the blue image size, but the green image size keeps corrupted

Notes:
- This issue appears both on Windows and OSX
- Removing or disabling the CanvasScaler Component of Canvas 2 should not affect elements of Canvas 1
- Undo functionality should work always, it should properly undo both, the green and blue image sizes
- If you select corrupted UI, set "UI Scale Mode" to "Constant Pixel Size" or "Constant Physical Size" and then reset it to "Scale With Screen Size", everything fixes up
- Setting Canvas.scaleFactor of a child canvas will scale the parent canvas

Reproduced on Unity 5.6.5p4, 2017.1.3p4, 2017.2.3f1, 2017.3.2f1, 2017.4.2f2, 2018.1.0f2, 2018.2.0b3 and 2018.3.0a1