shader: https://www.shadertoy.com/view/flcSRB
Thanks for the report. This has been fixed and ships in iOS15.2 and some upcoming macOS release. Most likely this is bug 230472 or bug 226660. Resolving as bug 226660 with a guess that the MSL shader fails due to name conflict with GLSL shader function name "rand". *** This bug has been marked as a duplicate of bug 226660 ***