So what happened?
This play comes up every few years, so we refer to the 2023 season's case of a fair infield single in New York, when HP Umpire Lance Barrett ruled Royals batter Bobby Witt Jr's bunt a fair ball after Yankees 3B DJ LeMahieu swiped his glove at the ball in fair territory.
Related Post: Why Was This Ball Called Fair in New York? (7/23/23).
There are two main standards for judging fair/foul: situations that consider whether a ball is on fair territory and situations that consider whether a ball is over fair territory.
A ball must be on fair territory to be considered fair in the case of: a batted ball settling between home and first/third base or first falling on fair territory beyond first/third base (e.g., in the outfield).
A ball must be over fair territory to be considered fair (whether or not it also touches [or is on] fair territory) for: a batted ball bounding to the outfield past first/third base, a batted ball that touches an umpire or player, or passes out of the playing field in flight (home run).
And, finally, a batted ball that touches first/second/third base is a fair ball.
Eagle-eyed viewers might also notice the rulebook's definition of foul ball is extremely similar, just substituting the phrase "foul territory" for "fair territory."
Thus we must also consider the following definitions: "FAIR TERRITORY is that part of the playing field within, and including the first base and third base lines, from home base to the bottom of the playing field fence and perpendicularly upwards. All foul lines are in fair territory"; "FOUL TERRITORY is that part of the playing field outside the first and third base lines extended to the fence and perpendicularly upwards."
Finally, the MLB Umpire Manual interprets FAIR BALL as, "When in contact with the ground, a ball must be in contact with fair territory and not merely over fair territory in order to be adjudged to be fair."
MLBUM added this interpretation following the introduction of Replay Review ahead of the 2014 season, and it works well for outfield fair/foul decisions on reviewable plays in which the ball may or may not fall on the foul line, generally observed from a parallax-prone camera angle.
For the infield fair/foul decision regarding a batted ball that contacts a person, however, a few key differences arise. First, this is the infield not the outfield. Second, this is not presently a reviewable play. Third, the rulebook criteria for this infield play remains on or over fair territory, as opposed to the outfield play's on foul territory criterion. Fourth, the umpire's angle in calling this play generally positions their head above the foul line, not entirely "top-down" but at an angle greater than 45-degrees and, thus, closer to top-down than ground level.
And herein lies the problem: The umpire, looking from above the height of the ball (unless they were to place their head directly on the dirt), generally will see a baseball over fair territory, all else equal, if any part of the baseball is over the foul line, even if the baseball itself is not in contact with the line (it's a sphere, after all). Pursuant to the Official Baseball Rules, the umpire calls this a fair ball when touched by the fielder, which conflicts with the MLBUM interpretation, which itself conflicts with OBR. Lovely.