diff --git a/src/Forms/GroupedDropdownField.php b/src/Forms/GroupedDropdownField.php
index 2dfd1e974..feb758361 100644
--- a/src/Forms/GroupedDropdownField.php
+++ b/src/Forms/GroupedDropdownField.php
@@ -2,6 +2,7 @@
 
 namespace SilverStripe\Forms;
 
+use SilverStripe\ORM\ArrayLib;
 use SilverStripe\ORM\ArrayList;
 use SilverStripe\View\ArrayData;
 
@@ -104,4 +105,14 @@ class GroupedDropdownField extends DropdownField
         );
         return $values;
     }
+
+    /**
+     * @return SingleLookupField
+     */
+    public function performReadonlyTransformation()
+    {
+        $field = parent::performReadonlyTransformation();
+        $field->setSource(ArrayLib::flatten($this->getSource()));
+        return $field;
+    }
 }
diff --git a/src/View/Shortcodes/EmbedShortcodeProvider.php b/src/View/Shortcodes/EmbedShortcodeProvider.php
index ba7fabc4f..f19529337 100644
--- a/src/View/Shortcodes/EmbedShortcodeProvider.php
+++ b/src/View/Shortcodes/EmbedShortcodeProvider.php
@@ -160,6 +160,17 @@ class EmbedShortcodeProvider implements ShortcodeHandler
             $arguments['style'] = 'width: ' . intval($arguments['width']) . 'px;';
         }
 
+        // override iframe dimension attributes provided by webservice with ones specified in shortcode arguments
+        foreach (['width', 'height'] as $attr) {
+            if (!($value = $arguments[$attr] ?? false)) {
+                continue;
+            }
+            foreach (['"', "'"] as $quote) {
+                $rx = "/(<iframe .*?)$attr=$quote([0-9]+)$quote([^>]+>)/";
+                $content = preg_replace($rx, "$1{$attr}={$quote}{$value}{$quote}$3", $content);
+            }
+        }
+
         $data = [
             'Arguments' => $arguments,
             'Attributes' => static::buildAttributeListFromArguments($arguments, ['width', 'height', 'url', 'caption']),
diff --git a/tests/php/Forms/GroupedDropdownFieldTest.php b/tests/php/Forms/GroupedDropdownFieldTest.php
index 47fb93fcc..fabb419b0 100644
--- a/tests/php/Forms/GroupedDropdownFieldTest.php
+++ b/tests/php/Forms/GroupedDropdownFieldTest.php
@@ -129,4 +129,39 @@ class GroupedDropdownFieldTest extends SapphireTest
             preg_replace('/\s+/', ' ', (string)$field->Field())
         );
     }
+
+    /**
+     * Test that readonly version of GroupedDropdownField displays all values
+     */
+    public function testReadonlyValue()
+    {
+        $field = GroupedDropdownField::create(
+            'Test',
+            'Testing',
+            [
+                "1" => "One",
+                "Group One" => [
+                    "2" => "Two",
+                    "3" => "Three"
+                ],
+                "Group Two" => [
+                    "4" => "Four"
+                ],
+            ]
+        );
+
+        // value on first level
+        $field->setValue("1");
+        $this->assertRegExp(
+            '/<span class="readonly" id="Test">One<\/span><input type="hidden" name="Test" value="1" \/>/',
+            (string)$field->performReadonlyTransformation()->Field()
+        );
+
+        // value on first level
+        $field->setValue("2");
+        $this->assertRegExp(
+            '/<span class="readonly" id="Test">Two<\/span><input type="hidden" name="Test" value="2" \/>/',
+            (string)$field->performReadonlyTransformation()->Field()
+        );
+    }
 }
diff --git a/tests/php/View/Shortcodes/EmbedShortcodeProviderTest.php b/tests/php/View/Shortcodes/EmbedShortcodeProviderTest.php
index 58ea889ed..379970c68 100644
--- a/tests/php/View/Shortcodes/EmbedShortcodeProviderTest.php
+++ b/tests/php/View/Shortcodes/EmbedShortcodeProviderTest.php
@@ -37,8 +37,8 @@ class EmbedShortcodeProviderTest extends SapphireTest
             [
                 'url' => static::$test_youtube,
                 'caption' => 'A nice video',
-                'width' => 480,
-                'height' => 360,
+                'width' => 777,
+                'height' => 437,
             ],
             [
                 'version' => '1.0',
@@ -58,7 +58,7 @@ class EmbedShortcodeProviderTest extends SapphireTest
         );
         $this->assertEqualIgnoringWhitespace(
             <<<EOS
-<div style="width: 480px;"><iframe width="480" height="270" src="https://www.youtube.com/embed/dM15HfUYwF0?feature=oembed" frameborder="0" allowfullscreen></iframe>
+<div style="width: 777px;"><iframe width="777" height="437" src="https://www.youtube.com/embed/dM15HfUYwF0?feature=oembed" frameborder="0" allowfullscreen></iframe>
 <p class="caption">A nice video</p></div>
 EOS
             ,