That does not tell my why it is an ID in this case but it is a field or group key in a different case.
The only think I can thing of in this that it’s a field key when loaded in JSON or created in PHP and a post ID when it is loaded from the DB.
I have looked and I don’t see any simple way to convert that ID into a Key.
You can try
$field_group = acf_get_field_group($id);
$key = $field_group['key'];
