To reproduce:
1) Open attached project
2) Make sure OpenGLES3 is enforced
3) Build and run on device or on 5.5+ game window preview in the editor with GLES3 emulation enabled
4) Notice that the Overlay Camera's view is rendered mostly black with some pink shapes
Tapping on bottom left button to switch the overlay camera to Orthographic mode fixes this

DUT:
Google Nexus 5 (Adreno 330)
Samsung Galaxy S7 (ARM Mali-T880MP12)
Samsung Galaxy S7 (Adreno 530)

Xiaomi Mi Pad (Nvidia Tegra K1-32) (Both GLESs' - it looks the same on OpenGLES2 like in above devices' OpenGLES3, and even worse in OpenGLES3)
Google Nexus 9 (Nvidia Tegra K1-64) (Both GLESs', but no additional noise on OpenGLES3 unlike Xiaomi)
Nvidia Shield Tablet (Nvidia Tegra K1-32) (Same as Google Nexus 9)