The problem was that the DBA had specified subschema IDMSNWKA to be used for DBAN.
Whenever running DBAN or Unloading/Reloading a dictionary area (DDLDML, DDLDCMSG, DDLDCLOD) use SS IDMSNWKU which includes all records and sets that can reside in these areas.
There is a note in the Utilities manual in the chapter on the UNLOAD UTILITY that is pertinent for DBAN as well:
When unloading a dictionary, the IDMSNWKU subschema should be used for all areas, except DDLCAT, DDLCATX, and DDLCATLOD.
The DDLCAT and DDLCATX areas should be unloaded and reloaded using the IDMSCATZ subschema.
The DDLCATLOD area should be unloaded and reloaded using the IDMSCATL subschema.