To reproduce:

1. Open Unity 5.5.3f1
2. Open attached project
3. Open "UItest" scene
4. You will see the UI showing up as intended
5. Close Unity
6. Open Unity 5.6.0f3
7. Open attached project
8. Open "UItest" scene

Expected result: When you open "UItest" scene, you will see all buttons with text, in a gray cube
Actual result: When you open "UItest" scene, you will see only a gray cube

Changing "Buttons" position in "Hierarchy" window doesn't fix the problem. If you create a new "Button" in "Grid" game object as a child, also do not shows the "Button". But if you create a new "Button" in "MenuPanel" game object as a child, you will see a fresh created "Button"

Reproduced on Unity 5.6.0f3. 2017.1.0a6
Not Reproduced on Unity 5.5.3f1
Regression on Unity 5.6.0b2