Reproduction steps:

1. Create a New project with one of the No Reproduction versions and import the "Mythic.zip" textures
2. Select the "Mythic-RedBird" Texture and see the default iOS texture settings ("ARGB 16 bit")
3. Drag that texture into the Scene
4. Set the Target Platform to "iOS"
5. See the Quality of the image
6. Open the Project with one of the Reproduction versions
7. Select the "Mythic-RedBird" Texture and see the default iOS texture settings have changed ("RGBA Compressed PVRTC 4 bits")
8. See that the image quality drops significantly

Expected Result: The quality of the image stays the same when the default texture settings are applied
Actual Result: The image quality drops significantly

Reproduced with: 2018.2.0b6, 2018.1.2f1, 2017.4.4f1, 2017.3.2f1, 2017.2.3f1, 2017.2.0a3
Did not reproduce on: 2017.2.0a2, 2017.1.4f1, 5.6.6f2

Note:
Issue reproduces in Android as well, but to a lower extent

Sometimes you have to override the texture settings and change them back to default, for the issue to appear/disappear