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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?

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TL;DR: it prefers to target ferrous metal

This is likely a hold-over from earlier editions, which had this text in the 2e Monster Manual entry for Rust Monster:

Rust monsters, being none too bright, stop pursuing a fleeing party for one round to devour metallic items, such as a handful of iron spikes, a mace or a hammer, if the party throws them behind. Rust monsters go after ferrous metals such as iron, steel, and magical steel alloys, such as mithril and adamantite. They choose such metals over valuable metals such as copper, gold, silver, or platinum. In fact, they would continue to pursue a party that just dropped a fistful of copper coins, for example, in hopes of getting the much-preferred ferrous metal of armor and weapons.

In 3rd edition, the Monster Manual has this text:

It prefers ferrous metals (steel or iron) over precious metals (such as gold or silver) but will devour the latter if given the opportunity.

(my emphasis)

Thus, the action describes a preferred target for the creature, being ferrous metals. It could well be that the ability might also corrode a non-ferrous metal .. but with 5e's "what the rule text says but nothing more" general principle, then it actually wouldn't, RAW.

So, generally, the statement "But seeing as bronze is not ferrous, it cannot be targeted" is erroneous, as it should really read "it won't be targeted".

There are also two "Ecology of the Rust Monster" articles in Dragon Magazine #88 and #346 that go more into the lore. Take what you read there with a pinch of salt, as some text contradicts earlier versions.

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It prefers ferrous metals

The Rust Monster description from a previous edition provides more details lore-wise:

It prefers ferrous metals (steel or iron) over precious metals (such as gold or silver) but will devour the latter if given the opportunity.

So it can corrode bronze, but normally it just doesn't want to.

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As far as known lore is concerned, there is no background explanation for this effect.

Page 28-29 of “Monstrous Supplement” of Planes of Law set states that rust monsters may be the larval forms oo rust dragons and evolve into them rarely in their old age. Also it suggests a nativity to the Plane of Acheron. This supports their possibly magical nature of their antennae attack, thus making it a supernatural effect rather than a chemical one.

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