sort_order not working on create

[Petoches]

Hello,

I can't find the solution, the sorting works well, but when i create a new data the sort_order is always set to 0.

    public $sortable = [
        'order_column_name' => 'sort_order',
        'sort_when_creating' => true,
        'sort_on_has_many' => false,
    ];

Thanks