I have some friends who are members in RWA, and rumors I've heard is that authors trying to sell romances with characters of color have frequently either been asked to change the race or outright told that the publisher doesn't sell "diverse books". The argument they give that books with characters of color (usually but not always black people) don't sell as well, but just like you, I haven't found that to be true at all in the self-publishing world. And I can't imagine the two markets are that different! I mean, I doubt most readers care (or know) about who published their favorite author.

Perhaps their data is just really out of date. Or I suppose it could be a self-sustaining cycle: a company sells only white books -> attracts a majority white audience specifically interested in white characters -> company believes that's the only thing that sells. Etc.