<datestamp> element not populated correctly when harvesting with marc21_withholdings prefix

Overview:
When harvesting with marc21_withholdings prefix datestamp element is set "1970-01-01T00:00:00Z". This is specially important during the incremental harvests when the datestamp shows when the record was modified.

Steps to Reproduce:

1. Harvest with marc21_withholdings prefix

Expected Results:
datestamp element is set to the instance modification date
Actual Results:
datestamp element always equals: 1970-01-01T00:00:00Z

Additional Information:
Works correctly for harvesting with marc21 prefix.

Record in inventory:

Harvested with marc21 prefix and marc21_withholdings prefix: