Koha doesn't have right authority frameworks to deal with free-floating subdivisions records 180, 181, 182, and, 185. These fields needs their own frameworks to work properly an to be searchable headings.

When you prepare this kind of records in ISO2709 file and export them to Koha, Koha treat well as free-floating subdivisions but there aren't frameworks to deal with. So the heading are stored but not searchable.

You can see this in module C4::AuthoritiesMarc lines 350-367


sub GuessAuthTypeCode {
    my ($record, $heading_fields) = @_;
    return unless defined $record;
    $heading_fields //= {
    "MARC21"=>{
        '100'=>{authtypecode=>'PERSO_NAME'},
        '110'=>{authtypecode=>'CORPO_NAME'},
        '111'=>{authtypecode=>'MEETI_NAME'},
        '130'=>{authtypecode=>'UNIF_TITLE'},
        '148'=>{authtypecode=>'CHRON_TERM'},
        '150'=>{authtypecode=>'TOPIC_TERM'},
        '151'=>{authtypecode=>'GEOGR_NAME'},
        '155'=>{authtypecode=>'GENRE/FORM'},
        '180'=>{authtypecode=>'GEN_SUBDIV'},
        '181'=>{authtypecode=>'GEO_SUBDIV'},
        '182'=>{authtypecode=>'CHRON_SUBD'},
        '185'=>{authtypecode=>'FORM_SUBD'},
},

NOTE: These records are not usable in biblio records but serves to librarians to take decision when creating complex-headings.