Reproduction steps:

1. Open "RectTransformBug.zip" project
2. Open "TestScene" scene
3. In the Hierarchy window observe "Canvas" -> "TestUI" and its children GO
4. In the "LeftPanel" GO enable "Aspect Ratio Fitter" component
5. Apply changes to the "TestUI" GO prefab
6. Delete the "TestUI" GO from the Hierarchy window
7. Drag the "TestUI" Prefab from the Project window into the Scene and child it to the "Canvas"

Expected Result: The "TestUI" looks like before Step 6
Actual Result: The "TestUI" "RightPanel" is no longer at the right edge (RectTransforms "Left" value is '-512' and "Right" is '512' instead of '0')

Fixed in: 2018.1.0b5

Reproduced with: 2018.1.0b2, 2018.1.0a7, 2017.3.0p1, 2017.3.0f1
Did not reproduce on: 2018.1.0a6, 2017.3.0b11, 2017.2.1p1

Note:
"LeftPanel" is anchored to the left edge
"RightPanel" is anchored to the right edge