How to reproduce:
1. Open the attached project
2. Check PlasmaBallTEX.tif texture in inspector
3. make sure that Android and iOS Overrides settings and Format is set to RGBA Compressed PVRTC 4 bits
4. Build and run on device or simply play in Editor
Result: Most right cube has black artefacts

Reproducible: 2018.1.9f1, 2018.2.2f1, 2018.3.0a2, 2018.3.0a7

Tested and reproduced:
[Vulkan/Metal, GLES3, GLES2]
Google Pixel XL 9 Snapdragon 821 MSM8996 Pro Adreno (TM) 530 OpenGL ES 3.2
Samsung Galaxy S9 8.0.0 Exynos 9 Series 9810 Mali-G72 OpenGL ES 3.2
iPhone X iOS 12.0
iPad Pro 10.5'' iOS 10.3.3
iPhone 6 plus iOS 8.0

By design: malformed image files did not define if alpha channel is premultiplied, causing bad mipmaps. Opening image file in any image editor and saving to the same format fixes the issue.

BY DESIGN: malformed image files did not define if alpha channel is premultiplied, causing bad mipmaps.