J.T. Realmuto hit a two-run home run to deep center field, at least in terms of the act. There is no question the ball cleared the wall and was a legitimate home run. Despite that, Realmuto ended up with only a RBI single and was ruled out after he mistakenly passed a teammate on the base path.
The issue occurred at first base. While Realmuto was heading to first watching the ball fly over the fence, Marcell Ozuna was scrambling back to first thinking he might need to tag up. Ozuna apparently wasn't sure if the ball would clear the fence or be caught. Realmuto appeared unaware he was even close to Ozuna and passed him as he rounded first. There was a moment of confusion before the two resumed trotting around the bases in correct order.
The Marlins celebrated, but the Brewers saw the gaffe at first and challenged the play. While the first base umpire didn't call anything live, umpires reviewed the play and determined Realmuto had passed Ozuna. Ozuna's run counted while Realmuto was credited with a single and ruled out at second base. He still was credited with one RBI, but his run comes off the board and his HR totals and slugging percentage take a hit.
And that is how you turn a home run into a single.