How to reproduce:
1. Download the user-submitted package (PrefabAndUmbra.unitypackage)
2. Create a new Unity project
3. Import the downloaded package
4. Make sure the active scene is 'SampleScene' and the Occlusion Culling mode is set to 'Visualize'
5. Open the Capsule prefab in Prefab Mode while having the Occlusion Culling window open

Expected result: the Prefab Mode is not affected by Occlusion Culling
Actual result: the Prefab Mode contains prefab-unrelated objects due to Occlusion Culling

Reproducible with: 2018.3.0a6, 2018.3.10f1, 2019.1.0b8, 2019.2.0a9
Could not test with: 2017.4.23f1, 2018.3.0a5(Old Prefab system is used in these versions)

Notes:
With Occlusion Culling Edit mode, prefab-unrelated objects do not appear in Prefab Mode, but the prefab is still culled depending on the Camera position
This Edit Mode behavior is still present in 2018.3.10f1, but is no longer reproducible in the newer branches since 2019.1.0a12 and 2019.2.0a1