Confirmed it was Metal.  Removed Metal from the player settings (so it'll use OpenGLCore like Unity 5.5), and now it works in 5.6.3.

Update: Apparently this issue only occurs on Mac's that have been updated to Sierra (Mac OS 10.12).  Sierra uses Metal for rendering.  Will enabling iOS Metal in the shader properties fix the issue?  I will try it when I can, but we're currently confirming that the prior version of Unity (5.5) does not have this issue, and re-importing assets takes a while.