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

Hi folks, following up on this, any update please? This is a major frustration for our customers so would be great to get it sorted ASAP