There are no steps for the behavior. I simply just spawn the npcs, I then go a little far away and when I come back they've despawned

All NPCs includes a custom name, a owner and removeWhenFarAway=false see:
https://github.com/MarkusBordihn/BOs-Easy-NPC/blob/1.20.1/Common%2Fsrc%2Fmain%2Fjava%2Fde%2Fmarkusbordihn%2Feasynpc%2Fentity%2FEasyNPCBaseEntity.java#L454

This means it's related to an optimization or performance mod which is not taking care of these flags.