One more point I want to add. From ECC 6.0 Ehp6 in the FM SAP_WAPI_DECISION_COMPLETE import parameter added for DECISION_NOTE.
According to new Note functionality, if the Note Justification is Required, we can use or not use the parameter DECISION_NOTE. It is working perfectly.
With Decision Note with Required justification attachment will create.
Without Decision Note, no Attachment will create.
But in case of Mandatory Justification in Note, it is always mandatory to write in the Note in case of execution by agent, otherwise it will not allow to complete the User Decision. But through this FM it is directly allowing to complete the step without passing anything to the parameter DECISION_NOTE.
It is not throwing an error. May be practically you are searching in production in technical log of workitem for Decision Note for Mandatory justification and you are not able to find out the attachment. Don't get surprise , it is possible to complete the Descision Step with Mandatory justification without any Note from SAP_WAPI_DECISION_COMPLETE.