Model's Prefab shape gets distorted when model Scale Factor value is changed, however it works for its Prefab Variant.

Steps to Reproduce:
1. Import a model to Unity Project.
2. Add model to Hierarchy.
3. Create a prefab by dragging Model GameObject from Hierarchy Window to Project Window.
4. Click "Original Prefab" on Create Prefab Prompt.
5. Select model in Project Window.
6. Right Click >> Create >> Select "Prefab Variant"
7. Drag Model Prefab and Prefab Variant to hierarchy.
8. Change Scale Factor of Model.

Observe: Prefab Variant gets scaled correctly and Model Prefab shape gets distorted.

Check Attached Video to reproduce the issue.

Reproducible On: 2018.3.0a2/e9113db2e9bf

Branch: prefabs/improved

Note:
- Appears on both OSX and Windows.