If you are editing a UI Object in Prefab Mode and attempt to add another UI Object as a child of the prefab, it will get added to the scene but will not be added to the Hierarchy of the prefab Object. You cannot select it and it will be discarded even if you save changes to the Prefab.

1. Install Unity
2. Open the attached project
3. Find the "UIImage" prefab in Assets and open it in Prefab Mode
4. Attempt to add a UI Image or any other UI object to the Prefab

Result:
Object appears in the Prefab Mode scene but cannot be selected and is not added to the Prefab Hierarchy.

Expected Result:
Object is added as child of the Prefab.