How to reproduce:
1. Contact me for the project, or setup SVN on a project with Prefabs and Settings
2. On computer one, make a change to a Prefab and commit it
3. On computer 2 update the project via Tortoise SVN and receive the updated Prefab
4. Outside of Unity open the Prefab in a text editor to confirm the changes have been sent properly
5. Also confirm the timestamp of the file has changed to show its been modified
6. Inside Unity, notice that the Prefab has not updated
7 .Hitting Ctrl+R to for a refresh, makes no difference
8. Right clicking the Prefab and re-importing makes no difference

Reproducible with: 2017.4.26f1
Not reproducible with: 2018.3.13f1, 2019.1.0f2, 2019.2.0a12

Actual results: Unity is caching the file change and not detecting that the original file has changed.
Expected results: Unity should detect file changes