public class ScrollToNewItemBehavior : Behavior<RadGridView>
    { 
        public static bool GetIsEnabled(DependencyObject obj) => (bool)obj.GetValue(IsEnabledProperty);
        public static void SetIsEnabled(DependencyObject obj, bool value) => obj.SetValue(IsEnabledProperty, value);
        public static readonly DependencyProperty IsEnabledProperty = DependencyProperty.RegisterAttached("IsEnabled", typeof(bool), typeof(ScrollToNewItemBehavior), new PropertyMetadata(false, OnIsEnabledChanged));
        private static void OnIsEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is RadGridView gridView)
                gridView.Items.CollectionChanged += (s, args) =>
                {
                    if (args.Action == NotifyCollectionChangedAction.Add)
                    {
                        gridView.ScrollIntoViewAsync(args.NewItems[0], gridView.Columns[0], null);

                        // exchanging args.NewItems[0] for gridView.Items[gridView.Items.Count-1] yields same result
                        //gridView.ScrollIntoViewAsync(gridView.Items[gridView.Items.Count-1], gridView.Columns[0], null);
                    }
                };
        } 
    }

    public class Message : ModelBase<Message>
    {
        public string Text { get; set; }
    }

        private RadObservableCollection<Message> _messages;
        public RadObservableCollection<Message> Messages
        {
            get => _messages;
            set
            {
                _messages = value;
                NotifyPropertyChanged(m => m.Messages);
            }
        }

            Task.Run(async () =>
            {
                for (int i = 0; ; i++)
                {
                    await Task.Delay(1000);
                    Messages.Add(new Message { Text = $"{i} - sftrvwj,erhvtwejhrfvtjlwehftrwejh" });
                }
            });

    <telerik:RadGridView
        x:Name="gridView"
        ItemsSource="{Binding Messages, Mode=OneWay}"
        IsSynchronizedWithCurrentItem="False"
        IsPropertyChangedAggregationEnabled="True"
        AutoGenerateColumns="False" 
        SelectionMode="Single"
        CanUserFreezeColumns="False"
        EnableColumnVirtualization="True"
        EnableRowVirtualization="True"
        CanUserDeleteRows="False"
        CanUserInsertRows="False"
        behaviours:ScrollToNewItemBehavior.IsEnabled="True"
        CanUserGroupColumns="False"
        IsReadOnly="True"
        IsManipulationEnabled="False"
        CanUserReorderColumns="False"
        CanUserSearch="False"
        ShowGroupPanel="False">
        <telerik:RadGridView.Columns>
            <telerik:GridViewDataColumn
                DataMemberBinding="{Binding Text}"
                Header="Text"
                ShowDistinctFilters="False"
                IsSortable="False"/>
        </telerik:RadGridView.Columns>
    </telerik:RadGridView>

        GroupRenderMode="Flat"