The Bucks lost to the Magic in the first game of their opening series. So they are certainly capable of losing to the Heat, who are much superior to the Magic and match up well to the Bucks. 
I envision a very tough series for Milwaukee, whose mental focus might not all be there. The Heat have playoff experience, added several strong pieces right before bubble play began and Erik Spoelstra is one of the better coaches in the league. 
The Heat have proven they can play with the big boys of the East going 8-5 versus the Bucks, Raptors, 76ers and Celtics. They dominated the Pacers going 7-1 against them. The Heat played tremendous in sweeping the Pacers in their first-round series winning all four games by nine or more points. 
Miami beat the Bucks in both of its meetings before the restart. The Bucks defeated the Heat, 130-116, in their lone bubble matchup. That score is highly misleading, though. Miami led by 17 points at halftime. The Bucks really wanted that game since they were off back-to-back losses and Miami didn't have Jimmy Butler and Goran Dragic. Now the Heat are healthy. 
The Heat are a dangerous foe to Milwaukee because of several reasons. Bam Adebayo could be the most improved player in the league. He's a tremendous defensive player and did one of the best jobs of any player in the league in defending Giannis Antetokoumpo. 
The Bucks' lone defensive weakness is 3-point defense because they stress inside defense stacking the paint. The Heat shoot 37.9 percent from 3-point range. That was second best in the NBA. 
Milwaukee led the NBA in scoring. The Bucks did it by being strong on the fast break and also scoring from inside the paint. The Heat gave up the fewest fast-break points in the NBA and also rank among the top-five in paint protection. 
So it's not some fluke that Miami beat Milwaukee two of three times.