When I make a report that exports to XSLX, then the newlines are being ignored in Excel. Example: http://cl.ly/fSNU
Strange thing is, when you make a minor change to the exported xlsx file and save it, when you open that file, new lines suddenly appear! So they are not actually lost.
This makes me believe JR is somehow not correctly formatting newlines for Excel.
I included a sample report that illustrates this problem.
Maarten @ Yesplan