Paper/paper-server/nms-patches/net/minecraft/world/effect/RegenerationMobEffect.patch

--- a/net/minecraft/world/effect/RegenerationMobEffect.java
+++ b/net/minecraft/world/effect/RegenerationMobEffect.java
@@ -11,7 +11,7 @@
     @Override
     public boolean applyEffectTick(EntityLiving entityliving, int i) {
         if (entityliving.getHealth() < entityliving.getMaxHealth()) {
-            entityliving.heal(1.0F);
+            entityliving.heal(1.0F, org.bukkit.event.entity.EntityRegainHealthEvent.RegainReason.MAGIC_REGEN); // CraftBukkit
         }
 
         return true;