Series tooltips are not visible in a Stock chart when the same series is added programmatically in the plotarea and the navigator simultaneously.

The workaround is to:
1) Create different series for the plot area and the navigator.
2) Explicitly set the tooltips' visibility with JS like this:
        <script type="text/javascript">
            function pageLoad() {
                var chart = $find("Chart");
                var series = chart._chartObject.options.series[0];
                series.tooltip.visible = true;
                //repaint the chart to apply the changes
                chart.repaint();
            }
        </script>