diff -u b/core/modules/text/src/Plugin/Field/FieldType/TextItemBase.php b/core/modules/text/src/Plugin/Field/FieldType/TextItemBase.php
--- b/core/modules/text/src/Plugin/Field/FieldType/TextItemBase.php
+++ b/core/modules/text/src/Plugin/Field/FieldType/TextItemBase.php
@@ -56,6 +56,10 @@
     $properties = $this->getProperties();
     if (isset($properties['format'])) {
       $format = $this->get('format')->getValue();
+
+      if ($format === filter_default_format()) {
+        $format = NULL;
+      }
     }
     return ($value === NULL || $value === '') && ($format === NULL || $format === '');
   }
only in patch2:
unchanged:
--- a/core/modules/field/tests/src/Functional/String/StringFieldTest.php
+++ b/core/modules/field/tests/src/Functional/String/StringFieldTest.php
@@ -94,7 +94,7 @@ public function _testTextfieldWidgets($field_type, $widget_type) {
     // Display creation form.
     $this->drupalGet('entity_test/add');
     $this->assertSession()->fieldValueEquals("{$field_name}[0][value]", '');
-    $this->assertSession()->fieldNotExists("{$field_name}[0][format]");
+    $this->assertSession()->fieldValueEquals("{$field_name}[0][format]", 'filtered_html');
     $this->assertSession()->responseContains(new FormattableMarkup('placeholder="A placeholder on @widget_type"', ['@widget_type' => $widget_type]));
 
     // Submit with some value.