Enemies jumping down from above, any type of ledge or ceiling, without height restriction, can jump into the ground vanishing completely. Either emerging from the ground or falling through if there is a open area below.
I’m not sure if this is geometry/location specific or it needs another kind of trigger.
Worst offenders are:
Jumping into ground and emerging, while being able to attack from below ground is the Skittergate end arena. Coming from the portal, the left corner with the metal plate. Enemies - any kind, from clanrat to monk -, after climbing up often jump into the ground and if player is close enough they can attack. If farther, they climb out of the ground.
Jumping down and falling through is Garden of Morr cauldron arena. Both places where the chains are on high ground and there is room below, enemies jumping from above fall through the ground and have to climb up from below. They cannot attack since it’s too high.
I noticed some other places, if this is location-based, let me know.