Paper/paper-server/nms-patches/net/minecraft/world/entity/monster/EntityVex.patch

--- a/net/minecraft/world/entity/monster/EntityVex.java
+++ b/net/minecraft/world/entity/monster/EntityVex.java
@@ -381,7 +381,7 @@
 
         @Override
         public void start() {
-            EntityVex.this.setTarget(EntityVex.this.owner.getTarget());
+            EntityVex.this.setTarget(EntityVex.this.owner.getTarget(), org.bukkit.event.entity.EntityTargetEvent.TargetReason.OWNER_ATTACKED_TARGET, true); // CraftBukkit
             super.start();
         }
     }