Stuck in “Mind Your Own Business”

I’m on the right side of the library portion of the quest. I’ve just done the jack in the box bit and finished fighting all the mannequins but one.

Every guide I’ve read says that I just need to kill the mannequins and then a door will pop up. However, one mannequin won’t die. Ancient magic, all my spells, petrificus totalas, nothing works. It still shows up as an enemy on my minimap but it doesn’t attack me or have a health bar. I’ve spent like 15 minutes beating the crap out of it to no avail.

Am I missing something or is this a bug?