I've managed to get to the root cause of this issue. It stems from the findNearestStackDatum.ts here.

  const { xScale, yScale, point } = nearestDatumArgs;
  const datum = (horizontal ? findNearestDatumY : findNearestDatumX)(nearestDatumArgs);
  const seriesDatum = datum?.index == null ? null : seriesData[datum.index];

datum?.index is not always reliable as it calculated based on the index of the item within the context of its own series.

So take the following

<BarStack>
  <BarSeries
    data={[
      { label: "first", value: 1 },
      { label: "second", value: 2 },
      { label: "third", value: 3 },
    ]}
  />
  <BarSeries
    data={[
      { label: "first", value: 2 },
      { label: "second", value: 4 },
      { label: "third", value: 6 },
    ]}
  />
  ;
</BarStack>;

This works fine, because when you hover over "second" it corresponds to index 1 in all data series. But if we were to do the following (which I believe is a valid use case)

<BarStack>
  <BarSeries
    data={[
      { label: "first", value: 1 },
      { label: "second", value: 2 },
      { label: "third", value: 3 },
    ]}
  />
  <BarSeries
    data={[
      { label: "first", value: 2 },
      { label: "third", value: 6 },
    ]}
  />
</BarStack>;

then "second" will reference index 1 in one data series, but return the "third" in the other as thats what index 1 is.

There is an obvious fix here to just fixup the data but I think that we could make the lib code a little bit more reliable by finding based on the stack and not the index. I have a fix working on my local so happy to push a PR.

I've updated my codesandbox with a little bit more context

I've opened a PR for this issue, would be great to get some feedback on it

Hi folks, would appreciate feedback on this PR as this issue is causing major frustration. Thanks