There’s a growing consensus that the 2017 MLB season featured a ball that behaved differently. The Ringer had the best breakdown of the belief in an overly-tightened baseball, resulting in a higher Coefficient of Restitution. In layman’s terms, the ball bounces more on contact with the bat. This led to the year of the home run, which saw more long balls in 2017 than any other season in baseball history.
Rob Manfred’s office has denied any change, but both published research and anecdotal evidence from players point to a different baseball, yielding questions about what ball will be used in 2018.