Autoscroll does not work correctly in DataGrid, how to rewrite behavior?

Question

WPF

In general, there is a task, there is a datagrid, SelectedIndex is set from outside it and you need something to autoscroll to the selected object immediately (well, as an addition, so that the next 5 objects can be seen). The whole problem is that the scroll does not reach the element that is far below the list, which is not visible now, for example, if I now see elements from 40-80, then it will not scroll automatically up to 120, but it will scroll up to 20.
Here is the Behavior code that I added to the datagrid

public class ScrollIntoViewBehavior : Behavior<DataGrid>
    {
        protected override void OnAttached()
        {
            AssociatedObject.SelectionChanged += new SelectionChangedEventHandler(AssociatedObject_SelectionChanged);
        }

        void AssociatedObject_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            var dataGrid = (sender as DataGrid);
            dataGrid.UpdateLayout();
            dataGrid.ScrollIntoView(dataGrid.SelectedItem);
            
        }
    }

Xaml inside DataGrid tag

<i:Interaction.Behaviors>
                    <behaviors:ScrollIntoViewBehavior />
                </i:Interaction.Behaviors>

And the index setting code

public void SetFocusOnCommandByNumber(int index)
        {
            if ((index >= _commands.Count) || (SelectedIndex == index))
                return;
            if ((index + 5) < _commands.Count)
            {
                SelectedIndex = index + 5;
                SelectedIndex = index;
            }
            else
            {
                SelectedIndex = index;
            }
        }

The index comes from outside several times per second, so I excluded change signals if the value did not change.