Ritual Altar Bugged

Bug report reference number 1,474,536,273

The third ritual altar isn't activating, despite there being 0 mobs remaining.

I have a feeling this bug might actually be literal. There's one of the cockroach thingies that came from the corpse when it died, and I can't kill it, but it's still running around (circled in red). Might be what's counting as a monster still active in the ritual circle?

Leaving & returning did not make the altar usable. Maven was watching the fight (tier 7 channel with niko mission on, and 2 sextants on the zone in the atlas).