To reproduce:
1. Open project attached by QA or download [Shader] FTP project from qa-project-share collab
2. Build Animated Compute Shader scene
3. Run it on mobile device

Reproduced with: iPad Pro 10.5' iOS 11.4, iPhone 7+ iOS 11.4.1, Samsung Galaxy S6 Edge+
Not reproducible with: iPhone X iOS 11.4, iPad Mini 2 iOS 9.2

Reproduced with: Unity 2018.3.0a6 (da73f99a4e02)

Doesn't work at all with Unity 2017.4.6f, 2018.1.3f1
Error: Metal: Error creating pipeline state (Hidden/InternalErrorShader): Vertex function requires input attributes but vertex descriptor has none
(null)

This shader that renders particles (points topology) do not output PSIZE from vprog which makes point size "undefined"
add to struct FragInput
float psize: PSIZE;

and in vprog
fragInput.psize = N;