The decline in Haaland's performance is due to several causes and it is impossible for a quality player to have decline in performance without there being cause and I don't know what the cause is for sure
The reason for Haaland's decline in performance is just because of De Bruyne's absence. Last season, when de Bruyne was active in Manchester City, he did create most of the opportunities that Haaland was always having, and we all know de Bruyne to be a talented player, he is always creating opportunities for Haaland, and we all know that Haaland is good at positioning himself and scoring goals. But since de Bruyne has been absent since the beginning of the season, Haaland performance has also dropped, he has not been scoring the way he was scoring last season, and nobody is giving Haaland passes the way de Bruyne do.