Minthara - Spider's Lyre

[GwynnApNudd]

Just wondering if anyone else has had trouble with the Ult for Minthara, Spider's Lyre.

The in-game description reads: "Minthara summons Kar'niss the Drider to attack all enemies. In a non-boss area, the Champions advance to the next area if no enemies remain after the attack."

The bolded part is where I'm confused - I often clear out every on-screen enemy with the Ult but I've only ever seen it skip the rest of that area once. Am I misunderstanding what qualifies as no enemies?