To reproduce:

1. Open the project, attached by the user (Reborn4.zip)
2. Open the "TestUIBug" scene
3. In Build settings, switch platform to Android
4. Enter Play mode
5. Select Image game object (child of Canvas) and observe that Default Sprite material was changed to ETC1 Supported UI material
6. Exit Play mode
7. Observe the image in the scene

Expected: the material doesn't show all the packed sprites, but shows only one that the UI Image has reference to
Actual: Instead of showing one sprite the ETC1 Supported UI material shows both packed sprites as one with the one that is supposed to be shown overlayed on top (see image in the edit)

Reproduced in 5.6.0a1, 5.6.3p1, 2017.1.0f3, 2017.1.0p4, 2017.2.0b8, 2017.3.0a4
Did not reproduce in 5.5.4p2
Regression introduced in 5.6.0a1

Fixed in 2018.1.0a1