i made, deleted, then tried to make again a simple rule with Cockpit -> Networking -> Edit rules and zone
Cockpit complained about the name i chose "custom--37916" on the re-make
i investigated in /etc/firewalld/services and the xml file for the "custom--37916" remained after i deleted it with Cockpit
not sure what's going on there, but wanted to report the incident. manual deletion of the xml file cleans it out fine