Currently when an Cloud instance is on the new billing systems, apps subscriptions and /or  app uninstall can only be performed on the Billing page of the instance.

https://support.atlassian.com/jira/kb/cant-uninstall-apps-after-cancellation/

A lot of customer are still not aware of this try to perform the actions in the UPM which results in an error. leading them to think that this is a bug (and end up either contacting Atlassian or the vendor of the app)

A notification should be added in the UPM to redirect the customers