Check_MK Discover always warning on failed check_mk@ services despite rule

These rules only apply if the services have already been discovered.
Could you check in "Setup → Services → Service Discovery Rules → Systemd Single Service Discovery” if there are any rules there?
The screenshots from the rules you posted are from “Setup → Services → Service Monitoring Rules → Systemd Single Services”
For what you are trying to achieve i don’t think you need those.