Adam Duvall caught Domingo Santana's dinger off the bounce, but it still wasn't an out

When a batted ball clears the outfield fence in between the foul poles and hits the stands, that's a home run. Everyone knows that. It's probably one of the most easily and widely understood rules in baseball.
If a ball has already become a home run, there is no reason to catch it if it bounces back onto the playing field. However, during Tuesday night's game between the Reds and Brewers, Reds left fielder Adam Duvall felt differently after Domingo Santana smashed a dinger in the first inning:

Despite Duvall's catch, Santana still rounded the bases to give the Brewers an early 3-0 lead and no outs were recorded on the play. At least Duvall was able to get some extra practice on his reactions and glove work.