When fighting in the <Predator of Vadabin> Black Rust habitat between the Nineshark and Black Rust habitats the main monster tends to de-aggro almost immediately after the first shot.

Steps to reproduce:

1) Sail to <Predator of Vadabin> Black Rust region

2) Shoot at a <Predator of Vadabin> Black Rust

3) Repeat a few times with different Black Rusts, about 40-60% of the time the monster will de-aggro and its babies will despawn instantly

Why this is a problem:

The baby Black Rusts that spawn are also valid loot targets for enhancement mats, so if about half of the time they despawn you're losing many chances to get those enhancement materials that make the zone worth doing.