When you attack a rust monster, the following may apply:
Rust Metal. Any nonmagical weapon made of metal that hits the rust monster corrodes. […]
So, bronze for example being a metal would corrode.
But the rust monster has the following action:
Antennae. The rust monster corrodes a nonmagical ferrous metal object it can see within 5 feet of it. […]
But seeing as bronze is not ferrous, it cannot be targeted.
What is the lore reason behind this?