LR initially displays the embedded preview or thumbnail and then interprets the full-resolution image and updates its thumbnail and preview with that data.
So whatever LR is showing initially is an interpretation of the embedded preview/thumbnail not a full-res image, and as such the 48-bit vs 24-bit situation is probably not in play, unless there are no thumbnails or previews embedded (which would be the scanner's fault) then LR could be doing something with the full-res data and not guessing correctly.
I don't think anyone can answer your question without an example DNG that has not been touched with LR, yet, and even then only someone with technical expertise in interpreting the contents of a DNG would be able to tell what is going on. The rest of us can just tell you how LR deals with the initial thumbnail/preview it shows as I did in my first sentence.
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Lightroom just displays the embedded preview, it doesn't generate it. As the embedded preview is always a JPEG, and JPEG is 8 bit/color by definition, it seems more likely that your scanning software is doing this, not Lightroom.
Ah! Should have realised that. Thanks