The named layer "country_polygons" in tegola.toml is assigned to admin_boundaries_11-12 and admin_boundaries_13-20 and while they do both come from the osm layer called "osm_admin_areas" it seems that in fact they are not areas, they are lines. Not only that but these layers contain far more than just country boundaries as they include all the admin levels available in osm from countries to states to cities to municipalities and more. Therefore, their named layer should be changed to something like "admin_lines."
The named layer "country_polygons" in tegola.toml is assigned to admin_boundaries_11-12 and admin_boundaries_13-20 and while they do both come from the osm layer called "osm_admin_areas" it seems that in fact they are not areas, they are lines. Not only that but these layers contain far more than just country boundaries as they include all the admin levels available in osm from countries to states to cities to municipalities and more. Therefore, their named layer should be changed to something like "admin_lines."