In a Domain Separated Instance you are finding that a Flow using the "Create Catalog Task" Action is getting stuck even though the Catalog Task has been set to Closed Complete.
This is happening because the Record Watcher [sys_rw_action] Records for the Catalog Task are being deleted unexpectedly and therefore the Flow is not made aware of the Catalog Task Closure and it cannot proceed as expected.
In this particular scenario the System Administrator [sys_user] Record had been moved into a different domain and it was causing this issue to occur.
Please check for any Scripts which would be deleting the Record Watcher [sys_rw_action] Records and ensure that this functionality is removed.
Also check the "FlowDesigner : Cleanup Record watcher" Scheduled Job since this will be deleting these records as per the Out Of Box Behaviour.
Since the Scheduled Job runs as the System Administrator User then it may be deleting unexpected records if the User Domain is set incorrectly.
You must ensure that the 'System Administrator' User is in the global Domain.