How to reproduce:
1. Create a new Project
2. Create a GameObject and drag it to the Project Window to create a Prefab out of it
3. Create another GameObject and create a Prefab out of it
4. Enter the Prefab Mode for the first GameObject and add the second GameObject as a PrefabChild of the first one
5. Enter the Prefab Mode for the second GameObject and create a few copies of the GameObject which is under that Prefab
6. Go back to the top of the Hierarchy

Reproducible with: 2018.3.0b7, 2018.3.0b9
Not Reproducible with: 2018.3.0b6

Expected result: adding or deleting GameObjects in the Prefab Mode has effect only within the Prefab in question
Actual result: adding or deleting GameObjects in the Prefab Mode creates instances of a Prefab in the Scene Hierarchy

Note: Improved Prefabs were introduced in 2018.3.0a6

Update: Fixed in 2018.3.0b10, 2019.1.0a1