It appears that when EPPlus creates the chart, it only stores the cell references for the data series. It doesn't actually store the value of each data point.
Microsoft Excel will recalculate the value of each data point (I believe), possibly ignoring any data point cache in the spreadsheet file.
OpenOffice/LibreOffice appears to read only the cached data point values in the chart section. This is why the chart doesn't appear in OpenOffice, because there are no data point values written via EPPlus.