My chocolate ghost go from green to brown, but I don't think that means the dealer did anything wrong. I think there are occasionally throw backs, not that you should throw them back. Just that sometimes a pepper will demonstrate its earlier origins. Have not looked into how the chocolate bhuts came about, but willing to bet the genes for brown existed in the red, met up and then selective seed saving came into play. Willing to bet some red dna stayed in there a bit. I've had the same thing happen, a red that should have been a brown. I also had a red that should have been a yellow. I just figure I either screwed up the tags or some of the former dna came to the surface.
Either way they taste good.