My bad !
When I updated the shaders to resync them with LL's develop viewer, I did not notice I was also adopting changes pertaining to the experimental PBR terrain painting feature, that I did not yet backport (too soon since still a moving target, and not even useful since only local to the viewer for now).
Apparently, the changes in the PBR terrain shader do not seem to impact modern OpenGL drivers (v4.6), but macOS can only do v4.1 or so...
Please, try this shader instead (replace the existing one in the installation directory of v1.32.2.30):