Bug report: Important bug in is_unique with DBGroup different than defined in $defaultGroup

[nowackipawel]

My database config is:

public $defaultGroup = 'defaultZT';

public $defaultZT = ['my old database credentials'];

public $defaultNEW = ['my new database credentials'];

and simple model class...

class AdminGroupModel extends \CodeIgniter\Model
{
	protected $DBGroup 		= 'defaultNEW';
	protected $table 		= 't_admin_group';
	protected $primaryKey 	= 'agr_identifier';
	protected $returnType 	= 'App\Entities\Admin\AdminGroup';

	protected $allowedFields = [
		'agr_identifier',
		'agr_description'
	];

	protected $validationRules = [
		'agr_identifier' 	=> 'required|min_length[1]|max_length[16]|regex_match[/[0-9a-zA-Z]{1,16}/]|is_unique[t_admin_group.agr_identifier,agr_identifier,{agr_identifier}]',
		'agr_description' 	=> 'required|max_length[512]',
	];
}

When I am trying to save (new \AdminGroupModel)->save($entity); i get an exception:

mysqli_sql_exception #1146
Table 'myolddatabasename.t_admin_group' doesn't exist

which obviously means it tried to connect my old database.... i noticed that validation rule is_unique causes this exception.