To reproduce:
            this.radScheduler1.ActiveViewType = Telerik.WinControls.UI.SchedulerViewType.Timeline;
            this.radScheduler1.RightToLeft = System.Windows.Forms.RightToLeft.Yes;

Please refer to the attached gif file. Select the left border of an appointment and try to resize it to the left. You will notice that you are allowed to resize it to the right and vice versa.

Workaround:           this.radScheduler1.SchedulerElement.ResizeBehavior = new MyResizingBehavior(this.radScheduler1.SchedulerElement);

public class MyResizingBehavior : AppointmentResizingBehavior
{
    RadScheduler scheduler;

    public MyResizingBehavior(SchedulerVisualElement activeOwner)
        : base(activeOwner)
    {
        scheduler = activeOwner.Scheduler;
    }

    protected override bool UpdateMouseCursor(Point mousePosition, Rectangle nearRect, Rectangle farRect)
    {
        bool result = base.UpdateMouseCursor(mousePosition, nearRect, farRect);
        if (scheduler.RightToLeft == RightToLeft.Yes )
        {
            FieldInfo leftFI = typeof(AppointmentResizingBehavior).GetField("leftResize", BindingFlags.NonPublic | BindingFlags.Instance);
            FieldInfo rightFI = typeof(AppointmentResizingBehavior).GetField("rightResize", BindingFlags.NonPublic | BindingFlags.Instance);

            if (nearRect.Contains(mousePosition) && scheduler.Cursor == Cursors.SizeWE)
            {
                leftFI.SetValue(this, false);
                rightFI.SetValue(this, true);
            }
            else if (farRect.Contains(mousePosition) && scheduler.Cursor == Cursors.SizeWE)
            {
                leftFI.SetValue(this, true);
                rightFI.SetValue(this, false);
            }
        }
        return result;
    }
}