diff --git a/C4/Biblio.pm b/C4/Biblio.pm
index 0fefd336dd..4f84b45bec 100644
--- a/C4/Biblio.pm
+++ b/C4/Biblio.pm
@@ -75,6 +75,7 @@ use Try::Tiny qw( catch try );
 use Encode;
 use List::MoreUtils qw( uniq );
 use MARC::Record;
+use MARC::Record::MiJ;
 use MARC::File::USMARC;
 use MARC::File::XML;
 use POSIX                     qw( strftime );
@@ -316,7 +317,10 @@ sub AddBiblio {
             my $marc_for_log = eval { $new_biblio->metadata->record_strip_nonxml };
             logaction(
                 "CATALOGUING", "ADD", $biblionumber, "biblio", undef,
-                { %{ $new_biblio->_unblessed_for_log }, _marc => _marc_record_to_diffable($marc_for_log) }
+                {
+                    %{ $new_biblio->_unblessed_for_log },
+                    _marc => ( $marc_for_log ? $marc_for_log->to_mij_structure : {} )
+                }
             );
         }
 
@@ -475,9 +479,12 @@ sub ModBiblio {
         my $updated_marc_log = eval { $updated_biblio->metadata->record_strip_nonxml };
         logaction(
             "CATALOGUING", "MODIFY", $biblionumber,
-            { %{ $updated_biblio->_unblessed_for_log }, _marc => _marc_record_to_diffable($updated_marc_log) },
+            {
+                %{ $updated_biblio->_unblessed_for_log },
+                _marc => ( $updated_marc_log ? $updated_marc_log->to_mij_structure : {} )
+            },
             undef,
-            { %{$original}, _marc => _marc_record_to_diffable($original_marc) }
+            { %{$original}, _marc => ( $original_marc ? $original_marc->to_mij_structure : {} ) }
         );
     }
 
@@ -502,37 +509,6 @@ MARC bib.
 
 =cut
 
-=head2 _marc_record_to_diffable
-
-  my $hashref = _marc_record_to_diffable($record);
-
-Converts a MARC::Record object into a plain hashref suitable for structural
-diffing. Control fields are stored as scalar values; data fields are stored
-as arrays of formatted strings containing indicators and subfields.
-
-=cut
-
-sub _marc_record_to_diffable {
-    my ($record) = @_;
-    return {} unless defined $record;
-    my %marc;
-    for my $field ( $record->fields ) {
-        my $tag = $field->tag;
-        if ( $field->is_control_field ) {
-            $marc{$tag} = $field->data;
-        } else {
-            my $ind1      = $field->indicator(1);
-            my $ind2      = $field->indicator(2);
-            my $formatted = "$ind1$ind2";
-            for my $subfield ( $field->subfields ) {
-                $formatted .= " \$$subfield->[0] $subfield->[1]";
-            }
-            push @{ $marc{$tag} }, $formatted;
-        }
-    }
-    return \%marc;
-}
-
 sub _strip_item_fields {
     my $record        = shift;
     my $frameworkcode = shift;
@@ -638,7 +614,7 @@ sub DelBiblio {
 
     logaction(
         "CATALOGUING", "DELETE", $biblionumber, "biblio", undef,
-        { %{ $biblio->_unblessed_for_log }, _marc => _marc_record_to_diffable($biblio_marc) }
+        { %{ $biblio->_unblessed_for_log }, _marc => ( $biblio_marc ? $biblio_marc->to_mij_structure : {} ) }
     ) if C4::Context->preference("CataloguingLog");
 
     Koha::BackgroundJob::BatchUpdateBiblioHoldsQueue->new->enqueue( { biblio_ids => [$biblionumber] } )
diff --git a/Koha/Biblio.pm b/Koha/Biblio.pm
index eff3a5f2c9..377251c345 100644
--- a/Koha/Biblio.pm
+++ b/Koha/Biblio.pm
@@ -2405,25 +2405,31 @@ sub merge_with {
 
 =head3 _unblessed_for_log
 
-Returns a plain hashref of biblio column values safe for JSON serialisation
-and action log diffing. Timestamp is excluded as it changes on every write,
-and undef/empty-string fields are removed so that genuinely unset columns
-do not produce noise in diffs.
+Returns a plain hashref of biblio column values that are not derived from
+the attached MARC record, for use in action log diffing.
+
+Fields such as C<author>, C<title>, C<subtitle> etc. are omitted because
+they are denormalised copies of MARC data and would duplicate what is
+already captured in the C<_marc> key of the log entry. Only fields that
+carry independent Koha metadata are retained: C<frameworkcode>,
+C<opac_suppressed>, and C<serial>. The C<timestamp> and C<datecreated>
+columns are also excluded as they are system-managed and add no signal.
 
 =cut
 
+# Biblio table columns that exist independently of the MARC record.
+my @_LOG_FIELDS = qw( frameworkcode opac_suppressed serial );
+
 sub _unblessed_for_log {
     my ($self) = @_;
-    my $data = $self->unblessed;
-    delete $data->{timestamp};
-    for my $key ( keys %$data ) {
-        if ( defined $data->{$key} && $data->{$key} ne '' ) {
-            $data->{$key} = "$data->{$key}" if blessed( $data->{$key} );
-        } else {
-            delete $data->{$key};
-        }
+    my $full = $self->unblessed;
+    my %data;
+    for my $key (@_LOG_FIELDS) {
+        next unless exists $full->{$key};
+        next unless defined $full->{$key} && $full->{$key} ne '';
+        $data{$key} = blessed( $full->{$key} ) ? "$full->{$key}" : $full->{$key};
     }
-    return $data;
+    return \%data;
 }
 
 =head3 _type
diff --git a/koha-tmpl/intranet-tmpl/prog/js/viewlog.js b/koha-tmpl/intranet-tmpl/prog/js/viewlog.js
index f0e1d1f332..399732f0e2 100644
--- a/koha-tmpl/intranet-tmpl/prog/js/viewlog.js
+++ b/koha-tmpl/intranet-tmpl/prog/js/viewlog.js
@@ -43,6 +43,14 @@ function renderStructDiff(raw) {
     function fmt(v) {
         if (v === undefined || v === null) return "";
         if (typeof v !== "object") return $("<span>").text(String(v)).html();
+        if (Array.isArray(v)) {
+            var items = v
+                .map(function (item) {
+                    return "<li>" + fmt(item) + "</li>";
+                })
+                .join("");
+            return '<ul class="diff-obj">' + items + "</ul>";
+        }
         var items = Object.keys(v)
             .map(function (k) {
                 return (