A document in status "draft" shows or requires an assignee. When document is published the assignee is empty, why this occurs?
Service Desk 14.1 or above
The behavior described is per design, the assignee exists when the document is being worked, in fact the document has a different id during this process, when the document is published again, there is no assignee to work on the document instead it has an owner and the id returns to the original document.
For example, document id 1001 is currently published, no assignee can be seen the database also should not have any value in the assignee field, if the document is reprocessed, the document id should change and then you will be ask to select an assignee. Once you finish and select to publish, you will be asked for typing an owner and then the original document id 1001 will return.