Arbitrary icons, Gramps and Gtk Inspector

Why does Gramps show B&W fallback icons when the color ones were specified? And how can it be made more predictable?

When documenting the interface, it is important to know whether to illustrate with the selected icon or the fallback. And whether it will be consistent across all the OSes.

The People category toolbar buttons (color and symbolic):

A more subtle example:

The hamburger icons that indicate that 2ndry objects exists have 4 stacked lines in the Linux Gnome GUI. (That icon is format-justify-fill-symbolic)

The Gtk Inspector says that the format-justify-fill (3 lines) was specified.