I don't know if this is a bug.
I found that if an export a download queue with a suffix *.com then it does not add the *.nzb suffix to the output NZB file. It seem to appear with the known file suffixes i.e. It works normally if the suffix is not a valid file type.
I can manually type the *.nzb suffix on without any problems and still have a valid NZB file.