Hi Tom,
This should never happen under any circumstance, so it is probably a bug. There is something in these images that yields NaN (Not a Number) values, which are represented as "#QNAN" by Microsoft runtime libraries.
Can you upload one of these images, so I can use it to find the origin of this problem? Also, it would be helpful if you could provide one of the original images (in a format different from XISF, such as FITS, TIFF, etc) that you used to generate the XISF files. Thank you, and sorry for the inconvenience.