In the most recent release (5.6.2f1) terrain grass flickers like crazy when rending via METAL. The frame debugger shows that this is happening during the RenderForward.RenderLoopJob step which I believe is applying shadows to the grass. Looks like the unity_ProbeVolumeMin is getting -infinity values:
Just discovered this, spent an hour trying to tweak things before it occurred to me to switch Metal off. Thanks for logging the bug and checking with QA!