All mobs dead. Suddenly, a mysterious frostbolt appears!

Bug report #: 685,497,862

/remaining says all mobs are dead yet it looks like an invisible Maelström is still casting frostbolt (or 'Maelström Projectile') at me from a fixed position at regular intervals. The frostbolts appear to originate from a beyond mini-orb...thing.

The frostbolt does trigger a hit, though doesn't seem to do any damage (to a naked character, no resistance over 20%).

I'm not sure what sequence of events led to this. I only noticed the orphaned frostbolt orbs after everything nearby was dead.

Tried:

• Killing all the mobs on the map
• Going 'out of range' (the other side of the map, more than 2 (5) screens away, enough to 'pause' the mob)
• Portaling out and back in
• 'Luring' the mob (it doesn't seem to move)
• Waiting 30 minutes. No change, frostbolts kept appearing every few seconds

Video:
https://www.youtube.com/watch?v=ytjDy5jp-Rc
Screenshot: