How to reproduce:
1. Open the "UI_order.zip" project
2. Open the "Menu" scene
3. Check the order of Canvas Base > Inner Area > child objects
4. Make sure the order is: Start Button, Fills, Transition, Audio Toggles
5. Press Play, the order of objects changes
6. (Optional) If the order doesn't change, swap the order of "Fills" and "Transition"

Actual result: The order of elements changes (gif attached).
Expected result: The order of elements doesn't change.

Reproducible with: 2018.1.0a1, 2017.3.0b5, 2017.2.0f3, 2017.1.2p1, 5.6.4p1, 5.5.5f1.

Note:
The order doesn't change if "StartButton" object is not the first in "Inner Area" child elements.
The order doesn't change if "Canvas Base" prefab is deleted from project assets.