The issue is that in the Socicons set of social icons (when building a widget) the icons for both Pixelfed and Vero are missing (they are missing anyway in any other icon set). The possibility to load a SVG icon is possible only when the plugin is used to add icon to a post or page, but not when buiding a widget (where the icons you can select are the only which are included in the plugin itself). Pixelfed and Vero are fast growing social, so I think the their icon should be added to the default set of social icons.