Ok, thomthom’s “Selection Toys” appears to be at least one of my installed plugins that breaks the Mac toolbar customization, and D.Bur’s “Axes Manager” is a second one. Still isolating potential others…
Interestingly, my own self-developed plugins break it 
Knowing this, however, I am going to look at my icon files and see if something correlates with what is in Selection Toys and in Axes Manager.