Lightroom is exporting Nikon NEF EXIF info such that the ISO rating will not display within Apple's Preview app info pane. However, after importing an NEF file into Photoshop through ACR, (latest 2015 CC version) then exporting to jpg, the ISO info will indeed show when that jpg is opened in Preview. Apparently Adobe has two methods of exporting EXIF info. This issue only happens with NEF files out of Lightroom. Other raw formats from Sony, Canon, and Lumix, are handled equally so that the ISO appears in exported jpg's opened with Preview. All other EXIF info seems to be included in exported NEF to JPG files. Any ideas?
The most efficient way to troubleshoot this is to upload a sample problem pic to Dropbox (or similar) and post the link here. Then we can take a look at what might be going on.
Sorry to not have replied sooner. I don't use dropbox but as Lightroom is a popular post-processing tool I was hoping someone using the Nikon .NEF (raw) format might join the conversation. The thing is, I can't figure out if this is a Preview or Lightroom issue.
Post up an offending conversion here - we'll be able to check whether we can see the ISO Exif in other viewers (or indeed, using an in-browser Exif reader), which will answer your question.