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Looking at the source code, it seems they are using ChartJs for the charts.
ChartJS should automatically find the right position for your tooltips, but since the chart area is so small, the tooltip would look cut-off when it has a top position too. Try increasing the area around the chart to give the tooltip some more space. Because all the other examples in your dashboard have larger charts/containers, the problem happens only in the small boxes.
When you want to have some more power over the tooltip, you can also switch to HTML tooltips so the tooltip will be added outside of the canvas, and can be configured through CSS (like Z-index).
Source:stackexchange.com