.../lib/Drupal/ckeditor/Plugin/ckeditor/plugin/Internal.php        |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/core/modules/ckeditor/lib/Drupal/ckeditor/Plugin/ckeditor/plugin/Internal.php b/core/modules/ckeditor/lib/Drupal/ckeditor/Plugin/ckeditor/plugin/Internal.php
index 5b8d9e2..1973790 100644
--- a/core/modules/ckeditor/lib/Drupal/ckeditor/Plugin/ckeditor/plugin/Internal.php
+++ b/core/modules/ckeditor/lib/Drupal/ckeditor/Plugin/ckeditor/plugin/Internal.php
@@ -296,6 +296,10 @@ protected function generateAllowedContentSetting(Editor $editor) {
     // Generate setting that accurately reflects allowed tags and attributes.
     else {
       $allowed_tags = filter_get_allowed_tags_by_format($editor->format);
+      // When all tags are allowed, also set allowedContent to true.
+      if ($allowed_tags === TRUE) {
+        return TRUE;
+      }
       $setting = array();
       foreach($allowed_tags as $tag => $attributes) {
         if ($attributes === TRUE) {