When the bubble chart is created and added label using the script, it gets generated in the slide. But once the ppt is saved the added label is removed and a junk label [CELL RANGE] will replace the added label. Adding the code snipped and the state of slide genereated below.
chart.ChartData.Series[0].Labels[index].DataLabelFormat.ShowLabelValueFromCell = true;
chart.ChartData.Series[0].Labels[index].ValueFromCell = workbook.GetCell(0, rowIndex, 0, cell.Title);
chart.ChartData.Series[0].Labels[index].DataLabelFormat.ShowValue = true;