When an instance uses the maximum number of licenses available and there is an add-on login present, which is a member of the atlassian-addons and the jira-user group, a license gets consumed.
The application license balance does not show a negative number.

The customer gets an license exceeded error and cannot create new issues.

Removing the application access (which will remove the login from jira-user) for those add-on logins helps temporary, but a restart of the instance will add those add-on logins again to the jira-user group, and the problem reoccurs.

This only appears to be the case with customer instances after the recent UUM upgrade.
Instances that already used the UMM don't appear to have this problem.