Issue #3466510: Fix custom date format in settings summary

src/Plugin/CustomElementsFieldFormatter/TimestampCeFieldFormatter.php

@@ -147,7 +147,7 @@ class TimestampCeFieldFormatter extends RawCeFieldFormatter {
   public function settingsSummary() {
     $summary = parent::settingsSummary();
     $date_format = $this->configuration['date_format'];
-    $date_format = $date_format === static::CUSTOM_DATE_FORMAT ? $this->configuration['date_format'] : $date_format;
+    $date_format = $date_format === static::CUSTOM_DATE_FORMAT ? $this->configuration['custom_date_format'] : $date_format;
     $summary[] = $this->t('Date format: @date_format', ['@date_format' => $date_format]);
     return $summary;
   }