Actually, I have kept chickens for years, and have had shell-less eggs too. It is like described above, it is more of a glitch in the egg making system, than a nutritional deficiency. I have at different times put out oyster shell, but really my hens will hardly touch it, and when I don’t have it out, their eggs are still fine.
It has just gone away in my flock, with or without oyster shell. I know it is a bit of a mess, but I am pretty sure it will go away pretty quickly. I know oyster shell is frequently advised, especially in mixed flocks where was is not feeding layer feed, but rather chick feed or grower feed. But I have not seen a clear correlation between good hard eggs and feeding more oyster shell, my birds will hardly touch it.