[Larry1953]

Bottom of 12th, one out, bases loaded. Cubs batter hits a slow chopper down the third base line. Astros F5 inexplicably tries to one hop it as it is going foul. He deflects it into fair ground - action about two-thirds of the way up the line. BU emphatically signals fair. The announcers (Cubs radio) say it is the PU call until it reaches the bag and the replay showed the Cubs caught a break.

Looking at the replay, PU was behind the plate, looking down the line but his view was blocked by the catcher and runner. My question is, how can the PU be responsible for both the anticipated banger at the plate and making the fair/foul call obstructed by the catcher and runner? Does the usual mechanic change when there is an R3?