Mob Spawners' Leash Does Not Work At All

Summary

Mob Spawners' Leash Does Not Work At All. The mob is not leashed back to the spawner.

Steps to reproduce

1. Create a spawner with any mob.
2. Try to get the mob out of the leash range (default is 32).
3. Get past that range (remember to use /mm s find 100 to get the exact range)
4. ???
5. Profit

Current behavior

The mob does not lose focus. When the mob has the ThreatTable module turned on, it's follow range is infinite, so the only good way to prevent the bosses from running away from the place of spawning is to use the leash mechanic. But it doesn't work.

Intended correct behavior

It should leash the mob.

Server log file

https://pastebin.com/xLWMBJ8y

Proposed fixes

No idea.