There are actually thousands of threads on this forum alone where plants are experiencing toxicity. Almost everyone assumes a deficiency, like you thought in your case. This, along with those few who claim that "EI causes no issues", gives the impression that toxicity is so rare that it's unlikely to be the case when it's the exact opposite. As for the studies, you'll have to know what search terms, spend dozens of hours reading, and be prepared to make analogous interpretations of the literature. Also, pay attention to those few of us who have done and are doing experiments into this. We have gone quiet for the aforementioned circus reason but we are still diligently experimenting. And discovering. What I've discovered so far makes me laugh at all the advice that is commonly given because almost all of it is wrong, even from those who seem most knowledgable.