The events are fired in the following order: UserAddingRow, EditorRequired, UserAddedRow eventfired when the new is pinned at the bottom and TAB key is processed.

Workaround:

public class FixGridNewRowBehavior : GridNewRowBehavior
{
    protected override bool ProcessTabKey(System.Windows.Forms.KeyEventArgs keys)
        {
            bool isInEditMode = this.EditorManager.IsInEditMode;
 
 
            if (isInEditMode)
            {
                IGridNavigator navigator = GridViewElement.Navigator;
 
                bool isLeftNavigation = navigator.IsFirstColumn(GridViewElement.CurrentColumn) && keys.Shift;
                bool isRightNavigation = navigator.IsLastColumn(GridViewElement.CurrentColumn) && !keys.Shift;
 
                if (isLeftNavigation || isRightNavigation)
                {
                    this.GridViewElement.EndEdit();
                }
 
                if (isLeftNavigation)
                {
                    navigator.SelectLastColumn();
                }
                else if (isRightNavigation)
                {
                    navigator.SelectFirstColumn();
                }
 
                if (isLeftNavigation || isRightNavigation)
                {
                    GridViewElement.BeginEdit();
                    return true;
                }
            }
 
            return base.ProcessTabKey(keys);
        }
} 

BaseGridBehavior gridBehavior = this.radGridView1.GridBehavior as BaseGridBehavior;
gridBehavior.UnregisterBehavior(typeof(GridViewNewRowInfo));
gridBehavior.RegisterBehavior(typeof(GridViewNewRowInfo), new FixGridNewRowBehavior());