diff --git a/C4/Biblio.pm b/C4/Biblio.pm
index ac35b38777..5c9dad5c6d 100644
--- a/C4/Biblio.pm
+++ b/C4/Biblio.pm
@@ -827,8 +827,17 @@ sub LinkBibHeadingsToAuthorities {
                         # to AddAuthority() itself is needed. Skip this when the heading's
                         # thesaurus couldn't actually be identified (bug 31925 comment 30) - in
                         # that case the site's own MARCAuthorityControlField008 default should
-                        # apply, the same as it always has, rather than being overridden.
-                        if ( Koha::Authority->has_known_thesaurus( $heading->{thesaurus} ) ) {
+                        # apply, the same as it always has, rather than being overridden. Also
+                        # skip it entirely when LinkerConsiderThesaurus is off (bug 31925 comment
+                        # 45): with thesaurus-aware matching disabled, headings from any thesaurus
+                        # can link to the same auto-created authority by text alone, so coding
+                        # that authority for whichever heading happened to create it would let its
+                        # thesaurus silently propagate onto unrelated bibs the next time the
+                        # authority is saved - only sites that opted into thesaurus-aware linking
+                        # (and so already search by thesaurus) should get thesaurus-aware creation.
+                        if (   C4::Context->preference('LinkerConsiderThesaurus')
+                            && Koha::Authority->has_known_thesaurus( $heading->{thesaurus} ) )
+                        {
                             my $date = POSIX::strftime( '%y%m%d', localtime );
                             $marcrecordauth->insert_fields_ordered(
                                 MARC::Field->new(
diff --git a/t/db_dependent/Biblio.t b/t/db_dependent/Biblio.t
index 5fda819a4e..50c5ee3981 100755
--- a/t/db_dependent/Biblio.t
+++ b/t/db_dependent/Biblio.t
@@ -1115,7 +1115,7 @@ subtest "LinkBibHeadingsToAuthorities record generation tests" => sub {
 };
 
 subtest "LinkBibHeadingsToAuthorities codes new authorities with the source thesaurus (bug 31925)" => sub {
-    plan tests => 11;
+    plan tests => 13;
 
     my $biblio_mod = Test::MockModule->new('C4::Linker::Default');
     $biblio_mod->mock(
@@ -1125,8 +1125,9 @@ subtest "LinkBibHeadingsToAuthorities codes new authorities with the source thes
         }
     );
 
-    t::lib::Mocks::mock_preference( 'marcflavour',           'MARC21' );
-    t::lib::Mocks::mock_preference( 'AutoCreateAuthorities', '1' );
+    t::lib::Mocks::mock_preference( 'marcflavour',             'MARC21' );
+    t::lib::Mocks::mock_preference( 'AutoCreateAuthorities',   '1' );
+    t::lib::Mocks::mock_preference( 'LinkerConsiderThesaurus', '1' );
 
     my $linker = C4::Linker::Default->new();
 
@@ -1200,6 +1201,29 @@ subtest "LinkBibHeadingsToAuthorities codes new authorities with the source thes
         'ind2=4 (source not specified) respects a customised syspref default 008/11=n, not forced to |'
     );
     is( $authority->subfield( '040', 'f' ), undef, 'ind2=4 (source not specified) gets no 040$f' );
+
+    # Case 7: LinkerConsiderThesaurus off (the default) - a FAST heading must NOT get
+    # thesaurus-aware coding on the new authority (bug 31925 comment 45). With thesaurus
+    # matching disabled, any other heading spelled the same can link to this same
+    # auto-created authority regardless of its own thesaurus, so coding it FAST here would
+    # let that thesaurus silently propagate onto unrelated bibs the next time the authority
+    # is saved. Only sites that opted into LinkerConsiderThesaurus should get this coding.
+    t::lib::Mocks::mock_preference( 'MARCAuthorityControlField008', '' );
+    t::lib::Mocks::mock_preference( 'LinkerConsiderThesaurus',      '0' );
+    $biblio = $builder->build_sample_biblio();
+    $record = $biblio->metadata->record;
+    $field  = MARC::Field->new( '650', '', '7', 'a' => 'Miracles', '2' => 'fast' );
+    $record->append_fields($field);
+    LinkBibHeadingsToAuthorities( $linker, $record, "", undef, 650 );
+    $authority = GetAuthority( $record->subfield( '650', '9' ) );
+    is(
+        substr( $authority->field('008')->data(), 11, 1 ), 'a',
+        'FAST heading (ind2=7, $2=fast) is coded with the syspref default, not z, when LinkerConsiderThesaurus is off'
+    );
+    is(
+        $authority->subfield( '040', 'f' ), undef,
+        'FAST heading gets no 040$f when LinkerConsiderThesaurus is off'
+    );
 };
 
 subtest 'autoControlNumber tests' => sub {