Are you sure they're laying the eggs in question or could they be laying somewhere you haven't found and/or not laying yet? All the BRs we've ever had lay a light brown almost pinkish egg and are easy to pick out vs other brown egg layers. We got a couple of Cuckoo Marans for some variety. Their eggs aren't the dark chocolate brown that I've seen in pics, but a brick or terra cotta red. We can easily pick them out from just the general brown eggs and no comparison to the BRs.
Are your's new to laying? I wasn't impressed with our pullet eggs, but they did darken up.