When you say "Multiple" batches how many is that exactly? I've raised ducks for about 5-ish years multiple ducklings and have never had any problems with Medicated Feed.No one can convince me that medicated isn't unsafe to ducklings. I've lost multiple batches of ducklings to medicated chick feed, and yes I have taken good care of them with a proper brooder, changed their shavings daily, made sure they had fresh food and water, and made sure that they were healthy. It might not always kill them, but IMO its best to be safe and not feed it to them.
It could also have been the brand and multiple other things.
This is just my opinion, but why risk your're ducklings or future mature ducks getting Coccia. when you can prevent it 100 percent? I also only feed medicated starter for 5-6 weeks then switch them on to Nutrena - All flock. I haven't had any problems with angel wing and all that stuff.