This is a:
What you did
I noticed that the Flash messages with general class "error" weren't being fixed up to "alert-danger".
So I updated line 72 of src/View/Helper/FlashHelper.php, adding an else for when it's not an array:
if (is_array($message['params']['class'])) {
$message['params']['class'][] = 'alert-' . $class;
} else {
$message['params']['class'] = 'alert-' . $class;
}
I really hope this helps.