PaperMC/paper-server/nms-patches/net/minecraft/server/AdvancementDataWorld.patch

--- a/net/minecraft/server/AdvancementDataWorld.java
+++ b/net/minecraft/server/AdvancementDataWorld.java
@@ -27,11 +27,15 @@
 import net.minecraft.util.profiling.GameProfilerFiller;
 import org.slf4j.Logger;
 
+// CraftBukkit start
+import java.util.HashMap;
+// CraftBukkit end
+
 public class AdvancementDataWorld extends ResourceDataJson {
 
     private static final Logger LOGGER = LogUtils.getLogger();
     public static final Gson GSON = (new GsonBuilder()).create();
-    public Map<MinecraftKey, AdvancementHolder> advancements = Map.of();
+    public Map<MinecraftKey, AdvancementHolder> advancements = new HashMap<>(); // CraftBukkit - SPIGOT-7734: mutable
     private AdvancementTree tree = new AdvancementTree();
     private final HolderLookup.a registries;
 
@@ -55,7 +59,7 @@
             }
 
         });
-        this.advancements = builder.buildOrThrow();
+        this.advancements = new HashMap<>(builder.buildOrThrow()); // CraftBukkit - SPIGOT-7734: mutable
         AdvancementTree advancementtree = new AdvancementTree();
 
         advancementtree.addAll(this.advancements.values());