Reproduction steps:
1. Open User's project
2. Switch target platform to iOS
3. In Graphics Settings switch Rendering Path to "Deferred" and turn on HDR
4. In Player Settings select Metal as main API
5. Build and deploy to an iOS device
6. Notice black squares on the screen

Expected behaviour: Identical behaviour on Metal and OpenGL ES.
Actual result: Graphics Artefacts on iOS with Metal API, HDR and Deferred Rendering Path;

Reproduced on versions: Unity 20117.2.0b1, 2017.2.0f3, 2017.2.1p3, 2017.3.1f1
Did not reproduce on: Unity 2017.1.3p1
Reproduced on device: iPhone 7 Plus iOS 10.3.2