From the implementation checklist chapter: Set up␠:ref:`MARC matching rules <record-matching-rules-label>` for importing records from mrc files or Z39.50 Matching rules are only used for imports, but not with Z39.50.
This is still valid for the 20.11 manual: https://koha-community.org/manual/20.11/en/html/implementation_checklist.html
Fixed by Eden