Big White Square on my photos

So I took some photos on my DSLR at the airport of planes. It was a cloudy day so after transferring to my Mac I edited the photos. Now all the photos I edited have a dirty white square on them. I revert the photos and it goes away.

What on earth has happened?

MacOS 10.15.3
Photos Version 5.0 (131.17.170)

I’ve had white blocks appear in windows when there was some sort of RAM cache corruption. Quitting apps (or restarting) always fixed it for me. But I never saw identical blocks in individual messages… unless the drawing routine effectively considers each image a ‘window’ of sorts.

So I’d restart my Mac and see if that solves it.

If not you might want to do an Apple Hardware Test to check the RAM integrity.