only in patch2:
unchanged:
--- a/src/Plugin/field_group/FieldGroupFormatter/Fieldset.php
+++ b/src/Plugin/field_group/FieldGroupFormatter/Fieldset.php
@@ -43,7 +43,7 @@ class Fieldset extends FieldGroupFormatterBase {
 
     $classes = $this->getClasses();
     if (!empty($classes)) {
-      $element['#attributes'] += ['class' => $classes];
+      $element['#attributes']['class'] = array_merge($element['#attributes']['class'], $classes);
     }
 
     if ($this->getSetting('required_fields')) {