That seems like an undesirable issue. I suggest you open a technical support ticket through our website.
A technician will be able to try and reproduce the issue, look with you for a workaround and report the issue to our R&D.
We have checked in the source code and there appears to be an issue with the sorting algorithm for XLS files. We will have to fix it in a future version.
In the meantime, if you save your file as a CSV (UTF-8) in Excel, then it works fine in the DataMapper.