I think Jaylen's flaws are still there. IMO, his game isn't entirely different than it was previously. The difference is the players around him. When you have KP and Jrue out there instead of Marcus and TL/Horford, it's a completely different situation than ever before. I do not believe that Jaylen and Marcus ever had a very good relationship, both on the floor and off. You can go back and look at the lineup numbers, and you can see clearly that when Jaylen was on the floor with Marcus but without Tatum or White, It wasn't good. At all.
The top 4 man lineup from last season was Tatum/Brown/Smart/Horford at over 800 minutes together. That group was a +1.4. Simply removing Smart from that group and replacing him with DWhite (a group with 717 minutes, 2nd most of any 4our man lineup), that number jumped to +11.7.
Jaylen is having his best year ever in terms of Assists, Turnovers, Turn%, and he's doing this on less 3's attempted, and much less usage. I believe he's making better decisions because he's trusting the guys around him more and because defenses can't load up on him the way they did previously whenever Jayson Tatum was off the floor. He doesn't need to be the #2 ball handler when Tatum goes to the bench anymore. He doesn't end up with grenades at the end of shot clocks because Marcus walked up the court for 10 seconds. He's not out there initiating PnR's with a guy that can't hit a 3 when the defenders collapse. He still has flaws, but unless Tatum rolls an ankle or KP gets hurt, I just don't think we see him get exposed in the playoffs like he did in Game 7 against Miami when he was forced into a role he's simply not good at. He doesn't need to be something he's not on this team.
And I still think people forget there is no Miami series, never mind a game 7 if Brown doesn't shoot 54% from the field and 47% from 3 in the first two series last year. It's easy to remember what happened in the Miami series, especially in game 7, but unless we see a rash of injuries, you won't see Jaylen Brown out there trying to initiate PnR's with Marcus/TL/Horford, where you have two guys that can't take a step back and hit a catch and shoot 3, and 2 guys who can't rim run.