Once ticket is with completed status, it still allows adding of comment in the notes facet.
SAP Cloud for Customer.
Reproducing the Issue
Go to the Service work center.
Go to the Tickets view.
Search for ticket LMN (where LMN represents ticket name with status completed).
Go to the Notes facet and click on the edit option.
You can see it allows you to add an internal comment.
In standard an adding of internal comment in completed ticket is allow and is an expected system behavior.
Internal Comment , Note facet , KBA , completed ticket , LOD-CRM-SRP , Service Request Processing , Problem