I think there's a lot of missing context here. First, remember all of Brown's "No comment" replies to the media last year? For example, he was asked why the team didn't have a lot of ball movement even though they were undefeated when they had something like 25 or more assists, and he said, "No comment." I can see Jaylen getting tired of playing with an inefficient ballhog. (I think that's what happened with Durant at OKC with Westbrook.)
Second, after the last win, Horford reportedly said that Jaylen had taken more vocal leadership in the locker room. It could be that the people telling SB that Jaylen is frustrated is just a function of him becoming a more vocal leader. Combine that with Bullpet losing sources in the front office, and you have your speculation that Jaylen is going to ask for a trade any day now.