How to reproduce:
1. Create a new project
2. Add Scroll View to the Hierarchy (right-click -> UI -> Scroll View)
3. Convert it to prefab by dragging it to the Assets
4. Click on Scroll View in the Hierarchy, go to Inspector and observe Overrides
Additional step if using Unity 2019.2.0a6:
5. Delete Scroll View object from Hierarchy, drag and drop Scroll View prefab from Assets to Hierarchy and observe Overrides

Expected results: No overrides
Actual results: Scroll View shows Viewport overrides

Reproducible: 2018.3.0a11, 2018.3.7f1, 2019.1.0b4, 2019.2.0a6
Not reproducible: 2018.3.0a6, 2018.3.0a10

Notes:
- The Prefab Mode was added in Unity 2018.3.0a6