I've always had the impression that JB has been a little envious of Tatum and doesn't like him. And honestly, I can understand it if he does. I mean, just look at the way both came in to the team:
JB:
1. Drafted #3 overall
2. Was essentially booed by fans when they drafted him - fans didn't believe in him
3. Was seen as a raw athletic prospect - came off the bench, had to earn every minute
4. Got very little press attention during his rookie year due to limited minutes
Tatum:
1. Also drafted #3 overall only one year later
2. Was mostly embraced by fans and viewed very positively
3. Got a starting role from day one and a lot of positive press
Tatum's rookie year the Kyrie and Hayward were lost for the season. and Brown and Tatum both stepped up huge to help carry the Celtics to the ECF and even put up a big fight against Lebron and the Cavs. Brown performed great, but Tatum performed just a tiny bit better, and so he got a lot of the credit/praise while JB was largely ignored.
This was basically the Celtics JB / JT tenure in a nutshell. Every year JB put in work, improved his game, got better. Every year JT was still just that wee bit better. So JT continued to get all the praise and awards while JB largely lived in his shadows. JB's name keeps popping up in trade rumors, while Tatum seems untouchable. Tatum makes the USA team, Brown gets ghosted.
JT continues to be the Celtic's golden child while JB is treated like 'the other Jay".
None of this is Tatum's fault. But it's human nature I think for any person in Brown's position to feel some amount of frustration and envy in a scenario like this, especially for a guy like Jaylen who is clearly the type of person who clearly a quite emotional / expressive type of perosn.
Then the Celtics win a title, JB gets FMVP, finally he gets his first taste of how it feels to be recognised. Then Tatum gets injured, misses the season, JB gets that spotlight and gets a bigger taste of that feeling of being the guy and getting all of the attention. For almost a full year JB actually starts to see what it feels like to be Jayson Tatum. Everyone is talking about the JB MVP push, etc.
Then Tatum returns just before the playoffs, and of course his return changes becomes THE story of the NBA. All attention is back on Tatum, Brown once again falls back into the shadows.
Do you think there was a part of JB that was kinda happy when Tatum got injured? I'm sure there was. Not because he's a bad guy or wants JT to suffer, but just because it's human nature for someone in this position to feel that way.
Then according to Brad, he spoke to JB in the offseason and briefed him on where things could be going. I assume he told JB that they really want him there, but couldn't offer him a supermax extension...and that if this is a dealbreaker they might need to consider other options. Then the Giannis trade blows over, and the Philly trade happens.
Now if you put yourself in the position of a guy who has gone through all over that over the past decade, do you think JB has some level of animosity towards Tatum? I'm sure he probably does.
That being said, I do genuinely believe Tatum when he says he has no beef or animosity towards JB. Firstly because Tatum probably doesn't have any reason to, and secondly because Tatum just doesn't seem like the type to get emotional and get caught up in drama - he seems to care about basketball and his family and not a whole lot else. I kinda feel like this is JB causing drama and finding his way to vent all that frustration out, and Tatum is probably just frustrated because he probably just thinks JB is being a drama queen.
It probably doesn't help that JB seems to have some element or arrogance to him. He does seem to have this opinion that he knows better than everyone else - he's always loved to lecture people about politics, so I'm sure he probably is the same way with other stuff. Would explain the whole "always likes to think he's the smartest guy in the room" thing and why Stephen A made the whole "not well liked around the league" comment. A guy like Tatum probably doesn't have a whole lot of patience for someone like that.