Mobs Despawn even though depsawn is set to false?

So I made a mob to execute a skill every 10 ticks but when I go away from it, it stops executing the skill pls help

RealismBot:
MobType: COW
Display: ''
Skills:
- potion{type=INVISIBILITY;duration=20000000;level=1} @Self ~onSpawn
- skill{s=MiningRealism} @PlayersInWorld ~onTimer:10
Options:
Silent: true
MovementSpeed: 0
Invincible: true
Persistent: true
Despawn: false