People are discussing Watchman as if it is a development of Mockingbird, when the truth is the other way around. Watchman may be set chronologically after Mockingbird, but Watchman was written first.

Secondly, and because this is a work of fiction, and because Watchman was shelved and never intended to be published (until now, for some reason I don't understand), I'm not sure that the Atticus Finch in Watchman really is the same person as the Atticus Finch in Mockingbird. Yes, they have the same name and were devised by the imagination of the same author, but every character was reshaped and reimagined from the never-to-be-published Watchman to the publisher-approved Mockingbird.

Thirdly, I strongly suspect this is a case of publicity-seeking cherry-picking. Nothing about this book is more guaranteed to get publicity than declaring the taken-to-heart hero turned out to be a racist. I'd like to read the whole novel in context before taking that bit of publicity on face value.