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Mobs in the Infinite Archive don't die if fall off the "arena", locking in Infinite Combat

Took me a while to figure out what happened because there were no visible mobs, and combat wasn't ending, then I paid attention to the red blip on the compass and found it.

Apparently, it was kicked off the "arena" by the maligraphic Watcher summon, so I didn't even notice it happening.