From what you describe, it seems that the camera-embedded preview JPG is intact while the raw data is partially missing for some of the photos. Files and Viewer are looking at the embedded JPG while LR is showing you the result of converting the raw data.
Try copying the files from the camera or card, again, import them with duplicates enabled, and see if the corruption is identical.
If the corruption is the same two times in a raw then the data on the card is corrupted and the problem is likely the card or occasionally the camera.
If the corruption is different between the two copy-import tries then the data is getting corrupted during or after the transfer and try using a different method to copy the files—a card reader vs camera and cable or at least try another USB cable and try copying to a different hard-drive if you have more than one as part of the experiment, to see if the drive is having issues.
This sort of issue is almost always the card but sometimes it’s not, so try to determine whether the card is bad or not. Besides seeing if the corruption is the same or changes across multiple copies/imports also try another card in the camera and shoot a bunch of photos and see if any of those are corrupted, and if any are, then the corruption is not specific to the card and is likely the camera or something during or after the copy is affecting things.
Thanks ssprengel. I'll check out your recommendations. ST
I disagree with the above assertion and say most of the time the issue is the card reader or cable, not the card. It can be ram but not usually.
Change the card reader or cable
Thanks Geoff. I did all of the above and was able to redownload the files without corruption.