png icon installed into share/icons/hicolor/512x512/apps/ isn't displayed in the menu

For example, the Jamulus project installs its icon into /usr/local/share/icons/hicolor/512x512/apps/jamulus.png and it isn't displayed with the menu item.

When I copy the same icon file to /usr/local/share/pixmaps/jamulus.png - it is displayed.

xfce4-desktop-4.14.3 FreeBSD 12.2