https://pastebin.com/EZukJ9Gz

Thanks! This might be because one of the creepers is disabled? Will fix this in the next update so it still spawns the creeper when disabled; also have some other fail saves in place to prevent this.

Don't worry, it was on a test world

Fixed in 1.2.1; it will however insta-despawn the creeper as it is technically disabled. Also update MultiMob please :)