Random colored blocks in RAW/DNG files usually point to file corruption rather than a Lightroom preview issue. Based on the answers, the first thing to suspect is the storage or transfer path—especially the memory card used when the images were captured/imported. A corrupted or fragmented SD card can cause this, and one responder noted similar issues after routinely deleting files from the card rather than reformatting it in camera.

Good first checks:

• Compare the same image across your backups to see whether corruption exists in every copy.
• Try a different memory card for new captures/imports.
• Reformat cards in-camera after safely backing up images instead of repeatedly deleting individual files.
• If corruption appears only in some copies, investigate the drive or transfer process.

Since the corruption is inside the DNGs, it’s likely a hardware/media or file-transfer issue somewhere between capture, import, and storage—not just catalog/previews.