Rails Admin 3.x doesn't work out of the box with Rails 8's Propshaft asset pipeline. The issue is that rails_admin's assets are SCSS files that need compilation, but Propshaft only serves static files—it doesn't compile SCSS like Sprockets did.
When you visit /admin, you'll see:
ActionView::Template::Error (The asset 'rails_admin.css' was not found in the load path.)