diff --git a/Bugzilla/Bug.pm b/Bugzilla/Bug.pm
index 9a84fb4467..7e382a1463 100644
--- a/Bugzilla/Bug.pm
+++ b/Bugzilla/Bug.pm
@@ -2700,7 +2700,9 @@ sub set_all {
 
   # And set custom fields.
   my @custom_fields
-    = grep { $_->type != FIELD_TYPE_EXTENSION } Bugzilla->active_custom_fields;
+    = grep { $_->type != FIELD_TYPE_EXTENSION } Bugzilla->active_custom_fields(
+        {product => $self->product_obj, component => $self->component_obj}
+    );
   foreach my $field (@custom_fields) {
     my $fname = $field->name;
     if (exists $params->{$fname}) {
diff --git a/Bugzilla/WebService/Bug.pm b/Bugzilla/WebService/Bug.pm
index 812a117ccd..fe5dda2b72 100644
--- a/Bugzilla/WebService/Bug.pm
+++ b/Bugzilla/WebService/Bug.pm
@@ -1731,20 +1731,26 @@ sub _bug_to_hash {
   foreach my $field (@custom_fields) {
     my $name = $field->name;
     next if !filter_wants($params, $name, ['default', 'custom']);
-    if ($field->type == FIELD_TYPE_BUG_ID) {
-      $item{$name} = $self->type('int', $bug->$name);
-    }
-    elsif ($field->type == FIELD_TYPE_DATETIME || $field->type == FIELD_TYPE_DATE) {
-      my $value = $bug->$name;
-      $item{$name} = defined($value) ? $self->type('dateTime', $value) : undef;
-    }
-    elsif ($field->type == FIELD_TYPE_MULTI_SELECT) {
-      my @values = map { $self->type('string', $_) } @{$bug->$name};
-      $item{$name} = \@values;
-    }
-    else {
-      $item{$name} = $self->type('string', $bug->$name);
-    }
+    $item{$name} = $self->_format_cf_value($field, $bug->$name);
+  }
+
+  # Include stored values for CFs not enabled for this product/component,
+  # so callers aren't silently missing data when a bug retains a value after
+  # its product/component changed.  Multi-select CFs are excluded: they are
+  # not preloaded by DB_COLUMNS and each accessor fires a separate SELECT,
+  # making them too expensive to include for hidden fields across many bugs.
+  my %seen_cf = map { $_->name => 1 } @custom_fields;
+  my @hidden_cfs = grep {
+    !$seen_cf{$_->name}
+      && $_->type != FIELD_TYPE_EXTENSION
+      && $_->type != FIELD_TYPE_MULTI_SELECT
+  } Bugzilla->active_custom_fields({skip_extensions => 1});
+  foreach my $field (@hidden_cfs) {
+    my $name = $field->name;
+    next if !filter_wants($params, $name, ['default', 'custom']);
+    my $raw = $bug->$name;
+    next if !defined($raw) || $raw eq '';
+    $item{$name} = $self->_format_cf_value($field, $raw);
   }
 
   # Timetracking fields are only sent if the user can see them.
@@ -1800,6 +1806,22 @@ sub _bug_to_hash {
   return \%item;
 }
 
+sub _format_cf_value {
+  my ($self, $field, $value) = @_;
+  if ($field->type == FIELD_TYPE_BUG_ID) {
+    return $self->type('int', $value);
+  }
+  elsif ($field->type == FIELD_TYPE_DATETIME || $field->type == FIELD_TYPE_DATE) {
+    return defined($value) ? $self->type('dateTime', $value) : undef;
+  }
+  elsif ($field->type == FIELD_TYPE_MULTI_SELECT) {
+    return [map { $self->type('string', $_) } @{$value}];
+  }
+  else {
+    return $self->type('string', $value);
+  }
+}
+
 sub _user_to_hash {
   my ($self, $user, $filters, $types, $prefix) = @_;
   my $item = filter $filters,
diff --git a/extensions/BMO/lib/Data.pm b/extensions/BMO/lib/Data.pm
index 2e2e8a893e..adf8b28afe 100644
--- a/extensions/BMO/lib/Data.pm
+++ b/extensions/BMO/lib/Data.pm
@@ -149,6 +149,7 @@ tie(
     "Testing"                                       => [],
     "Thunderbird"                                   => [],
     "Toolkit"                                       => [],
+    "Web Compatibility"                             => [],
     "WebExtensions"                                 => [],
   },
   qr/^cf_due_date$/ => {