Mobs Despawning After A Few Seconds? (Custom Mobs)

[KudaXD]

So I was trying to use MythicMobs on 1.12.2 to do bosses but when I spawn them they dont move and if you hit them its like their flying mid-air and then they disappear after a few seconds. We tried using another way of making bosses but same thing happened.

Anyone know a plugin or something that may be causing this?

 

[KudaXD]

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when and where can i find the location of when i asked?

 

[mayahicks]

I don't think you can control it going away. 90% of the time if it is removed all mobs will be removed except the tool created by Wikipedia.