How to call default interface implementation correctly?

the "override" of default interface impl of AcceptChanges does not call the default interface impl after casting this to interface type containing the default interface implementation. Why?

public interface IItemViewModel1
{
    void AcceptChanges() => Debug.Print("In interface");
}

public class ItemViewModelBase1<TModel> : IItemViewModel1
    where TModel : new()
{
    private TModel _data;

    public void AcceptChanges()
    {
        Debug.Print("In ViewModel");
        _data ??= new TModel();
        ((IItemViewModel1)this).AcceptChanges(); // calling itself rather than default interface implementation
    }
}

public static class TestClass
{
    public static void Test()
    {
        IItemViewModel1 x = new ItemViewModelBase1<int>();

        x.AcceptChanges();
    }
}