@Egg Snatcher If it helps, Nicalandia made a post on the allelic mutations of mottling a while back. Like- years ago. Some of the photos have since become unviewable.
https://www.backyardchickens.com/th...tling-and-its-many-allelic-mutations.1331859/
Someone else can probably explain it better, but just for sake of spitballing, I would guess you have 2 types of mottling hanging around? The thread Nicalandia started seemed to suggest D'uccle mottling was dominant over Japanese bantam mottling... I recall you started out your project with a brown red Japanese bantam x millie fleur D'uccle cross, which gave you mottled birds(not really possible unless the brown red Japanese carried mottling), so maybe that brown red rooster carried the japanese bantam type of mottling, and it's just been hiding in your line since, hidden by the D'uccle type?
I am not confident in what I'm saying.