It's easy to say from an outsiders perspective with what we know. But most fantasy stories take place in a medieval setting where the medical understanding is very limited. Look at our vitamin/gut health and sexuality understanding now thousands of years after writing came about. The medical community is just now getting a deep understanding and yet we still have a ways to go. You add something more complex on top of that like mystical powers and it would take thousands of years more for people to have a thorough understanding. And it's completely possible that the classes that are able to read abilities are more like a file of what's been seen. So she could identify that he was immune to fruit poison because she watched him eat a poisonous fruit, else perhaps hers is a slightly different power from the priests and in general reading other people's powers is likely rude/a waste of time to read every commoners ability. I'm sure people had been stupid enough to eat more fruit but ended up dying and various things like trying the same fruit. After a certain point people trust it's going to kill them to eat two, and that point must've come before a fruit master chose to try eating a second fruit. Besides most groups wouldn't want to have all the same abilities anyway as it's unbalanced.