There are different things that can cause soft shelled eggs. If you were consistently having soft shelled egg problems, then it would mean your flock is not getting enough calcium. Since that is not the case, that is not your problem. It still would not hurt and might help to offer oyster shell on the side. I know several members of this forum would mix oyster shell with the feed and try to trick all their chickens into eating an excess amount when most don't need it at the first sight of a soft shelled egg. I prefer offering it on the side where the ones that need it can get it and the ones that don't need it don't eat so much their internal organs have to work extra hard to get rid of the excess. Yeah, I know. In some ways I pamper my chickens. Whether the shells are hard or not, I offer oyster shell on the side. It lasts a real long time.
It is not unheard of for a hen to lay a second egg in a day. If her internal egg laying factory messes up, she can start a second yolk not that long after the first. Often when this happens, the shell gland does not have enough time to make enough shell material for the second egg, so it is soft or thin shelled. I don't think this your problem because of the size you mentioned.
When a chicken starts to lay, it sometimes takes a while for her to get all the kinks worked out of her internal laying factory. You might get shell-less, soft shelled, yolkless, nothing but yolk, or double yolked eggs. You can get all kinds of misshapen eggs. As complex as that internal laying factory is, I think it is remarkable as many get it right the first time as they do.
I'm not sure exactly what is going on with yours. Because of the size, I'd kind of expect your Sussex may have started laying, but I don't know. You can look at her vent and get a fairly good idea. If it is big, soft, and moist, she is probably laying. If her vent is small and dry, it is probably not her.
I personally wouyld not freak out and do rash things because of a rare soft shelled egg. If it is a consistent problem, then I would do something about it. I'll repeat myself. I see nothing wrong with offering oyster shell on the side whether they need it or not.