Barcelona and Manchester City usually play nice in the public eye. With the history between Pep and Barcelona there are usually some kind public words and even negotiations in the public eye.
However, according to reports, those negotiations weren’t always as amicable behind the scenes. The problem with dealing with someone you don't like is that if they have great players like City, you still have to deal with them.
In the case of Barcelona, they still had to work closely with City because they wanted Joao Cancelo and Ilkay Gundogan. There are also reports that City made some backchannel attempts at signing Barca youngsters like Lamine Yamal, Alejandro Balde and Pau Cubarsi.
Those efforts weren’t successful but it seems to have left a sour taste in Barcelona’s mouth, as all three players are considered to key for the club’s future after coming through the academy.