Nikolaj Coster-Waldau was on Jimmy Fallon a couple if weeks ago and said much of what the Kingslayer will be up to this season is not in the books.
The thing is there is a whole series of events that did not happen at all that did happen in the books that involved the Kingslayer and a couple of other characters.
What he has set out to do with Bron in the series is not in the books at all. I don’t know if that was to be in any books to be releaed later, but definitely not in books 1-5.