When using the component "minecraft:shooter", you can only target vanilla entities when the documentation states you can choose any entity.

In my custom entities, I've added the "minecraft:shooter" component, copied from a Ghast. I tried to change the definition to another entity I created that also has the "minecraft:projectile" component, but this change breaks my custom entity's behavior. As an example, there's a behaviors entity for fireball, dragon_fireball, arrow. I duplicated fireball, changed the identifier and referenced it in my custom entities behavior. This caused the custom entity to just stare at me when I got too close.

Alternatively, if I changed the definition in "Minecraft:shooter" back to "Minecraft:fireball" or "Minecraft:arrow", the custom entity shoots a fireball or arrow back at me, which is expected.

I've also made sure that my custom entity has the "Minecraft:behavior.ranged.attack" component.