I don’t know why many ballplayers burst onto the scene as rookies, announce themselves with a bang, and go through a protracted slump right after. Pitchers figure out you can hit a fastball and adjust, I guess. Ryan Mountcastle went through it, punishing the ball in 35 games as a rookie in 2020 (with a .333 average and .878 OPS) before faltering early the next April, slashing just .198/.229/.286 that month. He burst out of it, but it was still a trying stretch.
The latest guy to go through this is the preternaturally talented Gunnar Henderson.