How to reproduce:
1. Create a new Unity project
2. Create a 'Cube' and a 'Sphere' GameObjects
3. Turn them into prefabs
4. Pick one of the prefabs and drag onto the other one to replace that prefab
5. Go to 'Edit' and select 'Undo Create/Replace prefab'

Expected result: the replaced prefab is reverted to its previous state
Actual result: the previously replaced prefab is still identical to the one it was replaced by

Reproducible with: 2017.4.12f1, 2018.1.9f2, 2018.2.11f1, 2018.3.0b4, 2019.1.0a3

Notes:
In versions 2018.3.0a5 and earlier, undoing the replacement action and attempting to drag the prefab, that was the replacement, into the scene would display the warning "Unable to instantiate prefab. Prefab may be broken."