Infield Fly

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ASA slowpitch - Less than two outs.

Say you have infielders who are playing back on the grass setting up for a force play at any base. Pitcher is going to cover a base on force plays. A pop fly is hit to an infielder who is playing 30 to 40 feet onto the grass. They go back another 10 feet to catch the ball with routine effort but let it drop.

Is the still an infield fly and does the rule apply? If the infield was playing normal depth the ball drops in for a hit. While infielders are defined by their defensive areas around the bases they are really border line between infield and rovers but nothing in the rules defines this.

I've seen this and of course it get squawks by both sides depending upon the outcome.

What's the ruling I can hang my hat on? I can't find it in the rules....but that surely doesn't mean it's not there.