Steps to reproduce:

1) Download attached project 'ETC1_Bug.zip' and open in Unity.
2) Open scene 'test'.
3) File >> Build Settings >> Check if platform set to Android and Texture Compression set to 'Don't override'.
4) Click Play in Editor.
Note: Transparency of sprites looks fine.
See attached gif 'expectedsprite.gif'.
5) Quit from Play mode.
6) File >> Build Settings >> Set texture compression to ETC(default).
7) Click Play again.
Note: Animated sprites lost transparency.
See attached gif 'actual.gif'.

Expected result: Animations should not corrupt the Android + ETC compression transparency state.

Reproduced with: 5.2.4f1, 5.3.6p2, 5.4.0p1, 5.5.0a5