Paper/paper-server/patches/sources/net/minecraft/world/effect/OozingMobEffect.java.patch

--- a/net/minecraft/world/effect/OozingMobEffect.java
+++ b/net/minecraft/world/effect/OozingMobEffect.java
@@ -52,7 +52,7 @@
         if (entityslime != null) {
             entityslime.setSize(2, true);
             entityslime.moveTo(x, y, z, world.getRandom().nextFloat() * 360.0F, 0.0F);
-            world.addFreshEntity(entityslime);
+            world.addFreshEntity(entityslime, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason.POTION_EFFECT); // CraftBukkit
         }
     }