During the first season of laying, young hens are often getting the kinks worked out internally. I know next to nothing about Cochins, but I'd say heat stress combined with her youth could be factors. The fact that only one is affected could be genetics or that one individual not dealing well with the heat, or maybe getting picked on, or otherwise upset. Your feed seems fine, so I doubt that's it.
I've seen color rings in standard breed brown eggs similar to this, coupled with what I can only describe as rings of varying shell thickness, if that makes any sense. Those hens sometimes grew out of it, sometimes had to be culled if their eggs had thin areas resulting in frequent breakage. The bumpy shells also make me think of young hens, though some breeds seem more prone to laying them than others. I don't know about Cochins.
Perhaps someone smarter than me, or knowing more about Cochins will have a more definitive answer. I hope you get the answers you seek.