No there was never any choking that I noticed and when the ducklings died they weren’t near the feeder… and I’ve used the same food with all my duckingsHmm. That rules out some of the obvious. I could maybe see the airvent being a problem if it was blowing hot air on them or heating the area around them. But if you have been monitoring the temperature, that is unlikely.
Did you notice any excessive head shaking or any gasping? I had chick crumble recently that was unusually hard and a duckling choked on it.
I also read about someone being given the wrong feed from a feed store. It was a layer feed, and it killed several of her ducklings and other baby fowl. She had to call the manufacturer to figure out it was the feed killing hers. If the label is not obvious on the feed, it would probably be worth just double checking.