@Giovanni Derosas @Michael Rees Thelen, Nico Kindly note that the issue has been fixed by the product team. Thanks for confirming from your end as well.
Cause : The machines listed for a Maintenance configuration are shown based upon the response to an ARG query. Due to a problem with ARG these machines are not being returned in the results, so the machines do not appear in the UI. However, the machines were connected with the maintenance configurations and any scheduled patch jobs for that configuration will complete as expected.
Product Team has mitigated the issue, now the machines will appear in the Portal UI as well.