I was able to reproduce the bug, although I am still not sure what exactly causes it.
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I am fairly confident it has to do with gravity pull (the status).
1) In the screenshot below, the AI is in gravity pull status (see upper right corner) even though none of its skellies are under grav pull.
2) The AI's invicibility ended when one of its skellies got grav pull'd by my chaos mantle.
3) This roughly matches Shadow's explanations in 2 out of 3 cases (does not explain the sundial one, but may have been a coincidence)
(https://i.imgur.com/AjqdUy3.png)
----
Edit: My theory is that Chaos Mantle somehow applies gravity pull to a creature that is no longer in play, and therefore cannot die.
Hypothetically:
1) Opponent attacks with Skeleton
2) Chaos Mantle bolts Skeleton
3) Skeleton attacks again thanks to Writ of Vindication
4) Chaos Mantle grav pulls "ghost" Skeleton
5) From that point on, game redirects damage to opponent to an undefined creature with N/A HP