My three hens have been laying normal eggs since November. They get non-gmo, non-soy organic feed to which I add garlic powder, and dried oregano. I add apple cider vinegar to their water once a week. They also get oyster shell, cabbage leaves, apple rinds, pineapple husks, etc and get to free range for about 45 minutes a day. The past 3 or 4 days, one of the eggs has had a tiny soft spot on the top of otherwise normal egg. It is slightly lighter in color and breaks if rubbed gently.
For some reason one of your hens has developed a glitch in the 'shell forming' process of her egg production. It may be stress related, diet related (calcium shortage), or an indication that she is about to take a break from egg laying. It may very resolve within the next several days.