[1.11.2] WorldSavedData Not Saving

I have a custom WorldSavedData class but marking it dirty won't save it.

Where I edit values and mark it dirty:

	@SubscribeEvent
	public void entityDeath(LivingDeathEvent ev) {
		EntityLivingBase e = ev.getEntityLiving();

		if(!e.world.isRemote) {
			if(e instanceof EntityWolf) {
				WorldSavedDataMod data = WorldSavedDataMod.get(e.world);
				
				data.isDragonSlain = true;
				data.markDirty();
				
				...

			}
		}
	}

My WorldSavedData:

package com.kain.slippworld;

import net.minecraft.nbt.*;
import net.minecraft.world.*;
import net.minecraft.world.storage.*;

public class WorldSavedDataMod extends WorldSavedData {
	public static final String NAME = Reference.NAME + "_WorldData";

	public boolean isDragonSlain = false;

	public WorldSavedDataMod() {
		super(NAME);
	}

	@Override
	public void readFromNBT(NBTTagCompound nbt) {
		isDragonSlain = nbt.getBoolean(Reference.DRAGON_SLAIN_TAG);
	}

	@Override
	public NBTTagCompound writeToNBT(NBTTagCompound nbt) {
		nbt.setBoolean(Reference.DRAGON_SLAIN_TAG, isDragonSlain);

		return nbt;
	}

	public static WorldSavedDataMod get(World w) {
		MapStorage s = w.getMapStorage();
		WorldSavedDataMod d = (WorldSavedDataMod) s.getOrLoadData(WorldSavedDataMod.class, NAME);

		if(d == null) {
			d = new WorldSavedDataMod();
			s.setData(NAME, d);
		}

		return d;
	}
	
	public void save(World w) {
		w.getMapStorage().setData(NAME, this);
	}
}

When I kill a wolf it sets the boolean to true and it stays true, but when I exit the world and rejoin it's false.