How to reproduce:
1. Open the attached project ("1132706 - Prefab Canvas issues.zip")
2. Open the "Panel" prefab and add a Graphic Raycaster component to it
3. Observe the added Canvas component, save the prefab and leave the prefab mode
4. Reopen the "Panel" prefab again and observe the Canvas component, observe that it is now a "Root" type Canvas
5. Change Its "Render Mode" to "Screen Space - Overlay", save and leave the prefab mode
6. Observe that the Panel object's Canvas has changed back to the previous state and that the Image is completely broken

Expected result: the canvas component doesn't get changed to the "Root" type and doesn't cause further component breaks
Actual result: the canvas component changes to the "Root" type after saving the prefab and changing it to the "Screen Space - Overlay" mode causes it to break in the Editor

Reproduced in: 2019.2.0a8, 2019.1.0b6, 2018.3.8f1