One would imagine that they aren't going to clean the hammer mills,grinders, conveyors, augers, and pellet machines when switching over. There are lots of things in this world that have to be precise, exact, and contaminate free. Chicken feed, no matter the customers desires, simply is not one of these products requiring costly cleaning and maybe even sanitation.
The more involved a process is, the longer it takes to produce, the toll is running on overhead and labor. I could see where the first few bags might have some debris from the equipment or perhaps some stray product falling from an overhead conveyor.
I mean, even human food is allowed a certain percentages of mouse poop and insects. Few things in life can be done perfectly and fewer of us can afford perfection if it is attainable.
So here are two options for you.
Bust up a few random hand fulls of feed looking for the corn. If it is rare you have a choice to speak up and complain or stay quiet. Perhaps take the bag back for a refund. Then you face the feed store refunding your money and refusing to deal with you, or maybe the complaint gets back to the feed processor and the say "Fudge it." and just stop making the specialty feed as it attracts picky customers in their view.
Or, assuming there is minimal corn in the pellets, stay quiet and monitor the next bag to see if you got one of the first run bags with contamination.
And really research and question if a little corn is a big deal. For a lot of us it sounds like a first world problem. To you, obviously it isn't seen that way.