It wasn't always clear when your Suga URL was actually live. Adding a generated URL made the row in the dashboard look ready right away, even though the service still had to be redeployed before traffic flowed. Hitting a *.suga.run host that wasn't currently routed would land you on a generic gateway error, with nothing to say whether the route was wrong, the deploy had been torn down, or the platform was broken.
Both are sorted now. Custom domain rows show a clear pending state with a redeploy prompt right in the row, and unrouted *.suga.run hosts serve a branded Suga page that explains the hostname isn't routed yet.