Bacteria, viruses, and archaea currently have no iconic taxon and are pooled with unknown, see this feature request.
Another extra detail: there are actually two additional categories not displayed on the Explore filters – Chromista and Other Animals. So you can also find P. magnifica and your millipedes with this URL: https://www.inaturalist.org/observations?place_id=any&user_id=epic2112&verifiable=any&iconic_taxa=Animalia
Or by going to your Edit Observations page and choosing the Other Animals icon
Since Chromista is another iconic taxon, you can use the name instead of the number: https://www.inaturalist.org/observations?place_id=any&verifiable=any&iconic_taxa=Chromista