It’s pretty normal for a chicken to eat an egg that is broken. Maybe it fell and cracked or especially if the shell was very thin, just by walking on it or scratching to rearrange the nesting material in the nest. The problem is when a chicken learns to open them herself to eat them. That’s no real common but it does happen. It is possible a hen can teach other flock members to do that. I would not be overly concerned about finding one egg like that, but if a pattern develops, yes it is serious. I had a hen that would open them on purpose once. She would open them and a few others would help her eat it. I finally figured out which one was opening them and ate her. The problem went away.
There is another possibility. It may not be your chickens. There are a lot of animals that like to eat eggs and will leave the shell behind. Birds like ravens, crows, or magpies, rats, possum, skunk, raccoons, and more. I don’t know how tight you coop is, but it can be hard to keep everything out.
There are a lot of suggestions on how to break a hen from eating eggs if you really have an egg eater. The only one I tried was to eat the one that was doing it but I raise them mostly for meat so it was an easy choice for me. I don’t know how effective the other methods are since I haven’t tried them.
Good luck figuring it out. These things are not always easy.