The user exit EXIT_SAPMM06E_013 to update customer-specific data in the purchasing document is keyed while updating the document prior to the command COMMIT WORK. All the data related to the document update is then transferred.
The purchasing document number which is termed as EBElN is probably not filled in some of the tables that have been transferred through the interface of function module (XEKPO, XEKET). The document number is determined only after these internal tables are created when a purchasing document is created.
The standard system does not provide post-provisioning of the tables that have the documents. Therefore, when a document is created , it is possible that these tables are transferred along with the document number.