Reproduction Steps:
1. Open "Bug" project
2. Set "Edit > Project Settings > Quality > Texture Quality" to non " Full Res"
3. Notice that only the textures that are
- Texture is Square
- Side length is greater or equals to 8 pixels (8, 9, 10...)
- Side length is power of 2 (8, 16, 32, 64, 128, 256, 512...)
Only these textures are affected by this "Texture Quality" setting

Expected behaviour: All textures are effected by "Texture Quality".
Actual result: Only the textures with specific qualifications are affected.

Reproduced on MacOS and iOS.

Reproduced on versions: Unity 5.3.5f1, 5.3.7p4, 5.4.4p3, 5.5.0f3, 5.5.1p4, 5.6.0b9, 7.1.0a1
Reproduced on device: iPhone SE iOS 9.3, iPad Air iOS 8.1.1, iPhone 6s iOS 10.3
Did not reproduce on device: iPhone 4 iOS 7.1.2

Note: iPhone 4 iOS 7.1.2 was the only device that worked as expected.