If you save a bib record with a heading field (1xx, 6xx, etc.) containing a blank subfield a or other subfields but no subfield a, Koha incorrectly links the field to an authority record. It's not a blank authority, but a proper authority record of the same type (personal, topical etc.) but a nonmatching heading. It seems to target the same records each time, so in my system a blank personal name field (100, 600, etc.) always links to authority ID #1, for the Achaemenid dynasty. A blank X10 links to authority #48, for the Catholic Church, and so on. The situations described are cataloging errors in the bib record and so unwelcome, but they should not automatically link to valid but unrelated authorities. They should remain unlinked.