The Miami Heat are the worst team in the NBA against the spread. They are 6-12-1 on the season ATS, and 7-12 in wins and losses. Here's the thing. This team went to the NBA Finals last year. Only 4 teams have ever made the Finals one year and missed the playoffs entirely the next year. Those teams were the 1998 Bulls (Jordan retired, Pippen traded), 2004 Lakers (Shaq traded, Gary Payton traded), 2014 Heat (Lebron left, Bosh injured), and 2018 Cavaliers (Lebron left, again).
No major pieces from last year's team are gone. Injuries just hit at the wrong time. The team is 4-3 when Jimmy Butler plays this season. He just returned from injury on 1/30/21 and put up 30 points in a win against the Kings. 7 out of 19 games for Butler just isn't enough for the Heat to stay afloat. Now that he's back, the offense and defense should all function a bit better.
The only players who play significant minutes on their roster that have played in all 19 games are Duncan Robinson and Precious Achiuwa. Bam Adebayo has played 17 games, Tyler Herro 12 games, Goran Tragic 14 games, Kendrick Nunn 13 games, Avery Bradley 8 games, etc. Once there is a consistent rotation and their playmakers are back, the Heat will surge up the Eastern Conference standings.