I had seen that in the large fowl, although not for the past few years, but never in the bantams before. The odd large fowl grew up to be nothing worth keeping. I would cull these chicks and not waste any space, time or feed on them. My best guess is a different e-locus gene floating around in the gene pool. Either Eb or e+, and I lean toward Eb. Both are recessive to ER (birchen), which means unfortunately, that both male and female parents were/are carriers. Also means about 2/3 of the properly colored chicks would be carriers. If you want to rid your gene pool of this, you need to do a test mating on each of your breeders. Which might be a good reason to keep just a few of these oddities to use in test matings.