Not sure if the Bucks "figured them out" so much as the Bucks are just a much superior team.
Honestly have no idea what happened with the Heat last year. It feels unfair saying that their finals run was a "fluke" but it kinda feels like that's exactly what it was. They were a good team, they never really looked like a great one. It kinda seems like they just happened to click and go on a run at the right time, and as a result massively overachieved relatively to both their talent level and their regular season performance.
What I'm seeing from them this season is pretty much what I expected to see from them in the Playoffs last season.
I mean lets be honest, there isn't a true superstar on that Miami roster. Butler and Adebayo are All-Star calibre players but I wouldn't consider either of them a top 20 player. In fact I'd argue that Tatum and Brown are both better then those guys in terms of sheer talent. The difference is that Miami has a championship coach and has one of the greatest on-court leaders in the league (in Butler). They play as a team and they play hard, so they make the absolute most of the talent they have.