PaperMC/paper-server/patches/sources/net/minecraft/world/effect/RegenerationMobEffect.java.patch

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