How to reproduce:  
public partial class Form3 : Form
 {
     public Form3()
     {
         InitializeComponent();

         MapLayer pointLayer = new MapLayer("PointG");
         pointLayer.ClusterStrategy = new ElementClusterStrategy();
         pointLayer.ClusterDistance = 100;
         this.radMap1.Layers.Add(pointLayer);

         MapPin pin1 = new MapPin(new PointG(45d, 15d)) { BackColor = Color.Coral, ToolTipText = "Tool Tip1" };
         MapPin pin2 = new MapPin(new PointG(44d, 18d)) { BackColor = Color.Coral, ToolTipText = "Tool Tip2" };
         this.radMap1.Layers["PointG"].Add(pin1);
         this.radMap1.Layers["PointG"].Add(pin2);

         this.SetupProviders();
     }

     private void radMap1_ToolTipTextNeeded(object sender, Telerik.WinControls.ToolTipTextNeededEventArgs e)
     {
         MapCluster cluster = sender as MapCluster;
         if (cluster != null && cluster.ClusteredItems.Count == 1)
         {
             cluster.ToolTipText = ((MapPin)cluster.ClusteredItems[0]).ToolTipText;
         }
     }

     private void SetupProviders()
     {
         string cacheFolder = @"..\..\cache";
         BingRestMapProvider bingProvider = new BingRestMapProvider();
         bingProvider.Culture = System.Threading.Thread.CurrentThread.CurrentCulture;
         bingProvider.ImagerySet = ImagerySet.Road;
         bingProvider.UseSession = true;
         bingProvider.BingKey = "...";

         LocalFileCacheProvider cache = new LocalFileCacheProvider(cacheFolder);
         bingProvider.CacheProvider = cache;

         this.radMap1.MapElement.Providers.Add(bingProvider);

         bingProvider.InitializationComplete += bingProvider_InitializationComplete;
     }

     private void bingProvider_InitializationComplete(object sender, EventArgs e)
     {
         this.radMap1.BringIntoView(new PointG(45d, 15d), 5);
     }

 }

Workaround: handle the ToolTipNeededEvent
 private void radMap1_ToolTipTextNeeded(object sender, Telerik.WinControls.ToolTipTextNeededEventArgs e)
 {
     MapCluster cluster = sender as MapCluster;
     if (cluster != null && cluster.ClusteredItems.Count == 1)
     {
         cluster.ToolTipText = ((MapPin)cluster.ClusteredItems[0]).ToolTipText;
     }
 }