diff -u b/core/modules/text/src/Plugin/Field/FieldFormatter/TextTrimmedFormatter.php b/core/modules/text/src/Plugin/Field/FieldFormatter/TextTrimmedFormatter.php
--- b/core/modules/text/src/Plugin/Field/FieldFormatter/TextTrimmedFormatter.php
+++ b/core/modules/text/src/Plugin/Field/FieldFormatter/TextTrimmedFormatter.php
@@ -119,12 +119,15 @@
    * @see text_summary()
    */
   public static function preRenderSummary(array $element) {
+    $is_filtered = $element['#markup'] instanceof FilteredMarkup;
     $element['#markup'] = text_summary($element['#markup'], $element['#format'], $element['#text_summary_trim_length']);
-    // The markup will already have been filtered before getting here, but
+    // The markup could already have been filtered before getting here, but
     // if text_summary() trimmed it, it will be converted into a string
     // that will get autoescaped later, possibly removing markup allowed by the
     // text format. So, we make sure to convert it back into FilteredMarkup.
-    $element['#markup'] = FilteredMarkup::create($element['#markup']);
+    if ($is_filtered) {
+      $element['#markup'] = FilteredMarkup::create($element['#markup']);
+    }
     return $element;
   }