Mobs despawn onscreen

[Bumber64]

Describe the bug
Mobs despawn while still visible.

To Reproduce
Can happen whenever, but more noticeable if underground (lots of mobs) and using gold lantern with light potion (high visibility). Possibly exacerbated by fullscreen?

Expected behavior
Mobs should only despawn when far enough from the player.

Desktop

• OS: Windows 8.1
• Game version: 2.3.0-dev2