If documents are unexpectedly closing automatically, please check the "Status History" page for the documents. If they were closed at 12:00:00 by a user called "SYSTEM SYSTEM", then it is likely that the "Scheduled Date for Automated Closeout" field was populated on the "Closeout" page of the document before the document was released. 

When the specified date arrives, the document will automatically close at 12:00:00, and the user will be listed as SYSTEM SYSTEM on the "Status History" page.

To prevent these documents from being automatically closed, it should be communicated to the users who have access to the affected documents not to populate these values within the Closeout module.