To reproduce:

1. Create a new Unity project.
2. Create a Cube object in the scene.
3. Make it a prefab.
4. Modify some values of the cube in the scene(e.g Transform).
5. Revert Cube values to prefab values by pressing a gear icon-> Revert to Prefab on an edited component.
6. Undo the change by pressing CTRL + Z.

Expected result: Object values are reverted to values which were set before Reverting to Prefab.
Actual result: Object values stay the same as Prefab values. They are not undone to the previous state.

Fixed in: 2018.3.0a6

Reproduced with: 2018.1.0a2, 2017.3.0b7, 2017.2.0p1, 2017.1.2p2, 5.6.4p1, 5.5.5p1.