Deepworld is back online after a system outage. Thanks for your patience.
Sooooooo that stupid, rotten, no good disappearing mob issue seems truly fixed at this point. I've been poking around for five days now without seeing any issues.
Let's make sure the bug is gone, though! If you see a world with no mobs anywhere, post here ASAP and let me know the world name. Thanks, and good mob hunting!