Lightroom cannot convert a DNG file to a JPEG. That just cannot happen. I don't know what you are doing or what you are experiencing. But something isn't right. Can you still find the DNG files on your computer? Not necessarily in Lightroom, but are they still somewhere on your computer? If so, where are they? They have to be somewhere, because Lightroom cannot automatically change them.
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I've never heard about that. Are you shure that you doesn't have set any options that will be applied at import?
You can try to reset the Lightroom preferences.
Another thing to look at: Is your camera set to create both raw (DNG) and JPEG?
Or perhaps the original poster has set his camera mistakenly to JPG, but thinks the camera is actually shooting RAW photos.