During an BTE lookup submission, given this DOI 10.1161/CIRCULATIONAHA.117.030012 (source) , the retrieved record contains an ISSN, ISBN, and a DOI. The default mapping found under src/dspace/config/spring/api/BTE.xml under the bean "outputMap" by default maps ISSNs to dc.identifier.issn, ISBN to dc.identifier.isbn and DOI to dc.identifier.
Unless the proceeding input-form's describe steps contains fields that are specifically assigned to dc.identifier.issn and dc.identifier.isbn, the retrieved values that are a part of the lookup submission for those fields will not be imported and essentially discarded.
This is because the field to input-form validation logic defined under org.dspace.submit.lookup.DSpaceWorkspaceItemOutputGenerator lines 338-348 does not take into consideration the usage of fields that are qualdrops. Hence, in an input-form what is commonly referred to as an "Identifier" field which is generally of field type qualdrop, associated with the common_identifiers value-pair list found under src/dspace/config/input-forms.xml, would be populated with the retrieved DOI value by default but would not import the ISBN or ISSN into the input-form, thus losing that potential metadata.