A workaround is to force the tooltip to update its position when the content arrives (i.e., in the OnClientResponseEnd event: http://www.telerik.com/help/aspnet-ajax/tooltipmanager-client-side-on-response-end.html )

function OnClientResponseEnd(sender, args) {
    var activeTooltip = Telerik.Web.UI.RadToolTip.getCurrent();
    if (activeTooltip) {
        activeTooltip.updateLocation();
    }
}