I have the same thing happening from one of my girls too. Egg looks normal, then I wash it and the spots appear, then when it dries again they are almost invisible. I consistently have it one 1 egg. Not sure whose it is though. I figure it's just how she makes her eggs.
I was told if you put out the oyster shell into a separate container the ones who need it should eat it and the others won't, supposedly they know what it's for. I put out crushed egg shells (from my chickens, not store bought) in a separate dish for them. My girls eat it, but the cockerels...
These are our strangest eggs so far. We had a coyote incident & had a bunch of odd eggs for a while. Shell-less, soft shell. Not sure what happened w the yolk of the one. And the picture of the white egg next to the brown egg is the show size. The white egg is a store-bought large. I need to get...