Pixbuf missing in pygobject install?

I have installed Python3.7.2 and PyGObject GTK3 on MacOS 10.13.6, both with Homebrew.

Running the test gtk3-demo I can see there are images missing, and there are warnings:

(gtk3-demo:6501): Gtk-WARNING **: 11:33:01.313: Could not load a pixbuf from /org/gtk/libgtk/theme/Adwaita/assets/bullet-symbolic.svg. This may indicate that pixbuf loaders or the mime database could not be found.

As there is no /org directory installed, it looks like the entire images library is missing. This github comment from June 2017 seems relevant, but is not a solution.

Any suggestions on how to fix this please?

I really have exactly the same issue. All the column headers (created in glade just fine) is a black row in my GUI application due to this. Is there any solution available yet? Does anyone know how to install what is missing?