The plugin includes an icon font file (icon_fonts.css) with the default font-family name from IcoMoon. Because this loads in fairly late it will replace the font-family declaration for "icomoon" and replace any previously loaded icons. Unfortunately our theme (also 3rd party) uses the default name, but those icons are used in our main header and are much more obvious when the declaration is replaced and they go missing. I've put in a request for them too, but just using the custom font-family name option when exporting your IcoMoon icons would keep them from replacing previous icons. Thanks!