Oasis Mobs disappear when struck in water
 

Not 100% of the time, but I've replicated it multiple times. When Mobs are in the Oasis water (middle of the zone, not the ocean) and are hit, they will occasionally disappear. If they are chasing you, they don't reappear until they are right on top of you (free hit for the mob, really bad if you get stunned).

If they are fleeing they don't reappear until the re-agro.

It appears pets are not able to attack them either.

This happens around water and jagged ground. The enemy falls under the world. This happened a lot when I was fear kiting in Lake Rathe on my necro. The enemy would run into the water in a certain spot and disappear until fear dropped. They would pop back onto land when in melee range unless they were attempting to flee from low health.

Has also happened with the haunted chest near the beach in Cobalt Scar.

Have seen it now with Drakes and Wyverns in CS as well.