That's not suppose to happen!!
I know what the genetics experts will tell you. but crap happens. That could be a BR egg that got into the incubators miss marked and sold to you as a Dom.
I had a Legbar roost last year that hatched with a Pea Comb. The "experts" said it must have been crossed with something. It was not. My bird breeds never mix. Another Genetics person took the rooster to study it, he said things happen that aren't suppose to with just the right mix of genes. That could be the case here too.
It looks more like my Cuckoo Marans than my BRs though. Interesting Let us know what you find or discover.