Steps to reproduce:

1. Open attached project
2. Run 'Test' scene on Editor to see how it should look like
> There should be a label in top-left saying "Hello"
> And a button below which logs "Clicked" to console each time it is clicked
3. Build the 'Test' scene for iOS & deploy to device
4. Notice there is no OnGUI elements drawn, however, if you tap somewhere around the top-left corner, the button will be clicked and "Clicked" logged in the debug console

Does not reproduce on Android.

Reproduced with: 2017.1.0a1 rev 285a8e3fe401
Doesn't reproduce with: 5.6.0b8 rev ceacdcbcc360