That looks like a JPEG corruption. I can happen, if photos have transmission errors when downloading from the iPhone to the library, or if the internal drive has bad blocks and the photos get corrupted on the drive. It may also be a problem with the Graphic card.
Run a disk check to check your hardware.
Test, if the photos are only appearing damaged in the Photos library, but are correct if you export the unmodified originals from photos:
- Restart the Mac.
- Select one of the split files in Photos and use "File > Export > Export unmodified original".
- Now open the export photo in Preview. Is the exported photo undamaged?