to repeat, "Correlation is not Causation." Try again.
That explanation may satisfy you, but it lacks any plausible mechanism expected to result in such an outcome.
"Medicated" feed doesn't kill birds. Amprolium in the doses found in feed can't block enough Thiamine (B1) to matter (to them). Moreover, its easily reversible and hardly sudden in its effects. You'd have to ignore your birds clearly very sick behaviors for a lengthy period for them to die of...
According to Hubbard, this is a list of all their current feeds.
They do not currently offer a 20% protein Poultry feed. Neither do they appear to offer a generic "Livestock" feed of unknown nutritional values. Can you snap pictures of some of the other labels???
And FWIW I keep some generic...
I've looked quite closely at the feed label shown - nutritionally, there are no issues with the ingredients or the certified nutritional contents for chickens of any age.
Age of the feed could be at issue (molds, etc), a miss mix at the mill such that the contents of the package didn't match...