Soft egg found today
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That kind of egg is called a " Sac Egg." It is from lack of calcium. You should be feeding your hens some oyster shells and grit. To build up calcium in their diet. Or sometimes you just get one here or there from a healthy hen.
Here is a VERY helpful link about soft shelled eggs. You have to scroll down a bit on the page to find it. The Chicken Chick tells you everything you need to know
A one-off soft shell egg is nothing to worry about. If she continues to lay soft shell or shell-less eggs, then you'll want to inspect her fully (and probably the whole flock) for signs of mites, lice, worms, etc.
Sounds you are feeding them well. Be careful to not overdo it on treats. If this is your first time owning chickens, it can be exciting to feed them "special things". But many of the treats people like to feed are low in nutrition, usually not enough protein. Layer feed is usually 15-16% protein, which many consider to be the low end. I think the official absolute minimum protein for egg layers is 14%. And this may be for a specific commercial breed of egg layers, I'm not sure. If they get a lot low-protein treats, then that 16% protein from their feed can drop down much lower.