Unity's minimal UI shader problems with ETC1 alpha texture? 5.6

I'm using the following shader from this tutorial:

https://unity3d.com/es/learn/tutori...s/fill-rate-canvases-and-input?playlist=30089

Problems:

• Image UI element + ETC1 texture + tutorial shader = transparency outside in game mode, no transparency inside game mode.
• RawImage UI element + ETC1 texture + tutorial shader = everything correct.
• Image UI element + ETC1 texture + built-in UI ETC1 shader = weird blending outside game mode, correct display inside game mode.
• RawImage UI element + ETC1 texture + built-in UI ETC1 shader = no working transparency at all.

I need a perfomant UI shader that is compatible with ETC 4 bits + alpha and works with RawImage and Image elements.

I want to point out the texture has enabled alpha is transparency, alpha source is input texture alpha and is correctly atlased with a packing tag.

EDIT: Image format is .tiff just in case it has something to do.