I agree, doesn't look like a "normal" dwarfing going on with that one. It looks more like a much younger chick due to feathering- pictures makes it look like it still has some chick down attached plus still has chick feathers? The others have already long molted out to their adult shiny and...
Any pictures?
I have heard of dwarfism showing up in marans before. If I recall correctly, they also had short legs or something else off about them too.. is that how your bird looks?
It appeared to be something recessive as it would pop out of normal parents. If you want to test and see...