Steps to reproduce:

1. Import texture that has alpha information to Unity Project
2. Change Texture Type to 'Sprite (2D and UI)'.
3. Fill out any string to Packing Tag.
4. Change texture compression to 'High Quality'.
5. Open Sprite Packer, then click 'Pack'.
6. See compression format of packing texture, is BC7, but it has lost all alpha information.
you can look visually.

Expected outcome: Sprite is packed with alpha information intact
Actual outcome: Sprite loses alpha information, in this specific case it becomes entirely red

Reproduced on: 5.5.2f1,
Not reproduced on: 5.6.2f1, 5.5.4p1