My guess is that the files are corrupted—part of the raw data is missing. The slight curvature of the border is due to a lens profile being applied.
If the situation can be fixed by redownloading the file, then something must be happening in the transfer.
If this cannot always fix the situation then perhaps the card is bad and sometimes it works and sometimes it doesn’t. If downloading the file multiple times always results in exactly the same corruption then I’d guess the image is on the card that way. You can try using another way to download like Explorer or Finder, and see if the same situation happens.
If you want to upload one of the files that LR says is corrupted, others can test it to see if it is also bad on their systems. Sometimes a computer’s memory is bad on the image will only look bad in LR, but usually this manifests as things streaks and spots, not a block in the corner.
