Issuing ADAEND to a REPTOR should not be done before the connected source nuclei have ended.
When shutting down a production environment (typically before an IPL) - you can automate that the source nuclei (EG DB1,DB2,DB3) tasks have ended before ADAEND is issued to REPTOR.
In stead of depending on automation - change REPTOR ADAEND processing to wait for all connected source nuclei to disconnect. Or make a special command for this purpose.