Maintenance for the week of April 20:
• NA megaservers for patch maintenance – April 20, 3:00AM EDT (7:00 UTC) - 12:00PM EDT (16:00 UTC)
• EU megaservers for patch maintenance – April 20, 7:00 UTC (3:00AM EDT) - 17:00 UTC (12:00PM EDT)
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.