I understand the logic of iron becoming visible with broze working. Once a civ learns to work bronze, it's easy for them to identify iron ore deposits. And technically, they could have mined the iron.
But, mining iron should not unlock until iron working is discovered. Even if a pre-iron civ can technically mine iron, they don't know how to process it so they can't do anything with it. So why would a pre-iron civ waste resources mining useless iron when they could be doing other things instead? They wouldn't.
In the game you can plan ahead and mine iron so that once iron working is researched you can build swordsmen right away. But that's not how the real world works. it's not like the people in Italy in 2000 BC knew that the Romans would use iron for legionaries.