Playing around with the new lightroom and having files offline I discovered Lightroom will still export images even if the file is offline. Thats could be a problem when you’re expecting a full/high resoltuion image!
Primarily the smart preview is a small dng file and so won’t give you the resolution you are expecting if making a full size or similar image. I at first thought digital noise looked different, but it is about the same in both images.
to avoid the issue I suggest Jeffrey’s “Smart-Preview Support” Lightroom Plugin, It allows you to configure when exporting from the smart preview is ok and will bug you if you try and export from a smart preview when you don’t want to. The plugin is dontionware, so try it out and if you like it send Jeffrey a few bucks. He also has other great export tools as well as Lightroom productivity/file management plugins.