A rust monster can smell a metal object from up to 90 feet away. When it detects one, it dashes toward the source and attempts to strike it with its antennae. The creature is relentless, chasing characters over long distances if they still possess intact metal objects but usually ceasing its attacks to devour a freshly rusted meal. The creature targets the largest metal object available, striking first at armor, then at shields and smaller items. It prefers ferrous metals (steel or iron) over precious metals (such as gold or silver) but will devour the latter if given the opportunity.
The more accurate statement here has nothing to do with the 14-19 split and has everything to do with the level 14 characters getting access to creatures who drop from the "High" loot list. It wouldn't have mattered if they were all level 14.
I will admit that we made a mistake in allowing one of the under 20th levels to do the looting.