Reproduction steps:
1. Open the attached project "iOSTestProject-5.6.5"
2. Open the TestScene.unity;
3. Enter a play mode.

Expected results: all objects in the scene fade out smoothly;
Actual results: during fading out, grey artifacts start appearing.

The same behavior happens in the built project, grey artifacts start appearing even more.

Reproduced on: 5.6.0a1, 5.6.6f1, 2017.1.3p4, 2017.2.2p4, 2017.3.2f1, 2017.4.3f1, 2018.1.1f1, 2018.2.0b4, 2018.3.0a1.
Not reproduced on: 5.5.6f1.

Tested on: iPhone 5C (9.3.2) - same result as in the Editor,
iPhone 7 (10.3) - more grey artifacts than in the Editor,
iPhone 8 (11.0) - more grey artifacts.
Reproducible both on Metal and OGLES2 API.