There appears to be a difference in what Jaylen Brown says behind closed doors and what he divulges to Celtics president of operations Brad Stevens. Tracy McGrady, who has mentored the Boston superstar, said Tuesday that Brown is frustrated with the only team he’s ever played for his in his career, while Stevens said Wednesday during his end-of-season press conference that Brown has not revealed any lingering issues to him. “His frustration lies deeply in the organization and other things that we really don’t have the details. It’s just a lot of stuff I’ve been hearing just going on with the Boston organization with JB,” McGrady said Tuesday on his “Cousins” podcast with fellow ex-NBA star Vince Carter. Tracy McGrady said Brown is frustrated with the Celtics.…