diff --git a/core/modules/field_layout/src/FieldLayoutBuilder.php b/core/modules/field_layout/src/FieldLayoutBuilder.php
index 8ef5d048fc72f48a6ae563bb54bb72c79341f39d..fdbefd5e8d38c797a4b663daa3e950df829c54cd 100644
--- a/core/modules/field_layout/src/FieldLayoutBuilder.php
+++ b/core/modules/field_layout/src/FieldLayoutBuilder.php
@@ -67,12 +67,15 @@ public function buildView(array &$build, EntityDisplayWithLayoutInterface $displ
       $regions = array_fill_keys($layout_definition->getRegionNames(), []);
 
       foreach ($fields as $name => $field) {
-        // Move the field from the top-level of $build into a region-specific
-        // section.
+        // If the region is controlled by the layout, move the field from the
+        // top-level of $build into a region-specific section. Custom regions
+        // could be set by other code at run-time; these should be ignored.
         // @todo Ideally the array structure would remain unchanged, see
         //   https://www.drupal.org/node/2846393.
-        $regions[$field['region']][$name] = $build[$name];
-        unset($build[$name]);
+        if (isset($regions[$field['region']])) {
+          $regions[$field['region']][$name] = $build[$name];
+          unset($build[$name]);
+        }
       }
       // Ensure this will not conflict with any existing array elements by
       // prefixing with an underscore.
@@ -103,7 +106,7 @@ public function buildForm(array &$build, EntityDisplayWithLayoutInterface $displ
         // avoids breaking hook_form_alter() implementations by not actually
         // moving the field in the form structure. If a #group is already set,
         // do not overwrite it.
-        if (!isset($build[$name]['#group'])) {
+        if (isset($regions[$field['region']]) && !isset($build[$name]['#group'])) {
           $build[$name]['#group'] = $field['region'];
         }
       }
diff --git a/core/modules/field_layout/tests/src/Unit/FieldLayoutBuilderTest.php b/core/modules/field_layout/tests/src/Unit/FieldLayoutBuilderTest.php
index 8e6bf956c4bfde3c00b4ee0f36ebb79ff3099cfb..44332987cdcc9c8f17ef448d59ff1ad3b705288e 100644
--- a/core/modules/field_layout/tests/src/Unit/FieldLayoutBuilderTest.php
+++ b/core/modules/field_layout/tests/src/Unit/FieldLayoutBuilderTest.php
@@ -93,6 +93,9 @@ public function testBuildView() {
       'test1' => [
         '#markup' => 'Test1',
       ],
+      'test2' => [
+        '#markup' => 'Test2',
+      ],
       'non_configurable_field' => [
         '#markup' => 'Non-configurable',
       ],
@@ -111,6 +114,9 @@ public function testBuildView() {
       'test1' => [
         'region' => 'right',
       ],
+      'test2' => [
+        'region' => 'unknown_region',
+      ],
       'non_configurable_field' => [
         'region' => 'left',
       ],
@@ -120,6 +126,9 @@ public function testBuildView() {
     ]);
 
     $expected = [
+      'test2' => [
+        '#markup' => 'Test2',
+      ],
       'non_configurable_field' => [
         '#markup' => 'Non-configurable',
       ],
@@ -169,6 +178,9 @@ public function testBuildForm() {
         '#markup' => 'Test2',
         '#group' => 'existing_group',
       ],
+      'test3' => [
+        '#markup' => 'Test3',
+      ],
       'field_layout' => [
         '#markup' => 'Field created through the UI happens to be named "Layout"',
       ],
@@ -190,6 +202,9 @@ public function testBuildForm() {
       'test2' => [
         'region' => 'left',
       ],
+      'test3' => [
+        'region' => 'unknown_region',
+      ],
       'field_layout' => [
         'region' => 'right',
       ],
@@ -207,6 +222,9 @@ public function testBuildForm() {
         '#markup' => 'Test2',
         '#group' => 'existing_group',
       ],
+      'test3' => [
+        '#markup' => 'Test3',
+      ],
       'field_layout' => [
         '#markup' => 'Field created through the UI happens to be named "Layout"',
         '#group' => 'right',