I have this same problem with version 4.20.1. Even the Strapi is with that version, I see notification badge and that “Upgrade your admin panel” -text in the Overview -panel.
I have run npm run build before npm run develop, and it seems anyway to build it straight with the develop. But the notification still is there… does that mean it has not updated the admin panel? If so, how to do that? It does show the latest strapi version above it.