Using both LR Classic CC 7.3 to convert and the DNG Converter 10.3 to convert, I tested some Canon 5D Mark IV raw samples from the Imaging Resource, and I see no visible issues.
Please be specific as to what process you're using the convert to DNG, and what versions of what software you're using, and upload a sample CR2, preferably a new one before having been touched by any software, to somewhere like dropbox, google-drive or one-drive, and post a link, here. There may be firmware differences since the samples were posted in 2016.
Here are my results that don't show any corruption. These are from within LR after Import but the Import panel as you've pictured, also shows no corruption:
Corrupted thumbnails are usually due to corrupted raw data within the image file, or when importing, from corrupted previews, although I am seeing neither when I do my testing.
If your images are newly shot and have just been converted to DNG then where are the craw files when you convert them and where do you convert the DNGs to--the source and destination? If these are older images that were converted long ago, then probably the storage where they reside has become corrupted. None of those sorts of details where given in the first message, so I'm just speculating based on common issues people report.
the 2 files in the preview earlier. Several clarifying comments: this occurs randomly (but repeatedly on same file) whether straight from the card (files from 2 different cards), copied locally or on the NAS I use for long term storage.
Same behavior in LR Classic 2018 7.2, Photoshop (Camera Raw 10.3) and Adobe DNG converter. All under Win 10
my normal workflow is to import in LR, Copy as DNG with embedded original and 2nd copy file to a backup NAS.
typical behavior as you saw in the preview screenshot is for the import preview to look OK but at import, the DNG shows the artifact.
Same thing in CR, initial view looks OK but then CR evaluates the RAW and the artifact appears.
Maybe that suggests damage tot he underlying .CR2 but I did successfully convert to DNG using the stand alone DNG converter and then imported that file into LR. That has not worked on these 2 files.
I just noted that LR shows using CR 10.2, not 10.3. I'll have to look into why that did not update.
Also, I have tried turning off graphics acceleration in LR -seems no difference.
The bad news is the CR2s you shared already contain corrupted raw data before my trying to import into LR, as evidenced by opening one in RawDigger raw analysis software:
If the two CR2s you uploaded came from your NAS then you don't have any valid CR2s, unless you happen to have them in a third location, perhaps on the memory card.
Stop converting to DNG until you understand the issue.
I suspect the files are already corrupted before import, perhaps on the memory card or the copy from the memory card, but because the Import panel only shows the camera-embedded preview, which doesn't contain the corruption, only the raw data has corruption, viewing them on the Import grid is masking the problem until later in the process.
From now on, try importing the CR2s without converting to DNG and make sure all the LR-rendered thumbnails are ok before you use LR to convert to DNG from the LR [Library] / Library / Convert to DNG..., OR JUST LEAVE THEM AS CR2 FILES.
It's possible the corruption is coming from the copy on your NAS and LR is being efficient and copying the files to the NAS, then Converting to DNG from the NAS copy and that NAS file-i/o is getting corrupted, either to or from the NAS because something isn't 100% compatible.
It could merely be that you're copying from the memory card or the card, itself, are bad, but, again, because you're not viewing rendered raw data on the Import panel for the CR2s, you can't tell. The DNGs are corrupted on the Import panel because their raw data has been rendered by Adobe to create the DNG preview using Adobe default settings.
Finally, here are the two CR2 files, that are showing as corrupted in RawDigger or any other software that interprets the raw data to form the image, on the Import grid, not showing any corruption because the image, there, is clearly a low-res preview:
Unfortunately, DPP shows the camera-embedded preview until the last possible moment, but if you double-click to open the editor and make any adjustment that changes the look of the image, here I've adjusted the brightness up to 0.50, DPP will read the raw data to re-render the image and you'll see the bad spot:
The bottom line is that the bad data is in the CR2, already, and has nothing to do with LR or converting to DNG, only that because of the process you're using, the first time you see the rendered raw data is in the DNG preview so you've jumped to the conclusion that the DNG conversion process is the problem.
If you had imported the CR2 files into LR you'd see the problem or if you'd made adjustments in DPP you'd see the problem, both without converting to DNG.
I'm not sure what else to tell you than what I told the person just above that also has the problem, don't use the memory card you've been using and if you can, make a backup copy of the raw files in camera to a second memory card--if it has that mode, so at least you'll end up with one good copy, or if both copies are bad in camera then you'll know it's the camera doing something to the data before it's written out, but it's almost always the card, or something happening as you're copying from the camera to the computer, some iffy USB/Lightning connection, etc.
You can also turn on RAW+JPEG to eliminate the RAW rendering step on the computer. That way you have a finished JPEG to compare with your RAW processor.
the ACTUAL problem was the card reader (that I have used for thousands of files...) This morning, just by lucky accident, I put the card in a different reader and voila! clean file.
I then followed my routine workflow and the LR import also worked fine.
The obvious lesson learned is to look at the WHOLE chain and isolate each element. All of the contributions helped in that direction but I guess I'm satisfied that being lucky beats being good.
That veteran reader is now polluting a landfill.