--- a/net/minecraft/server/ServerAdvancementManager.java
+++ b/net/minecraft/server/ServerAdvancementManager.java
@@ -21,10 +21,14 @@
 import net.minecraft.util.profiling.ProfilerFiller;
 import org.slf4j.Logger;
 
+// CraftBukkit start
+import java.util.HashMap;
+// CraftBukkit end
+
 public class ServerAdvancementManager extends SimpleJsonResourceReloadListener<Advancement> {
 
     private static final Logger LOGGER = LogUtils.getLogger();
-    public Map<ResourceLocation, AdvancementHolder> advancements = Map.of();
+    public Map<ResourceLocation, AdvancementHolder> advancements = new HashMap<>(); // CraftBukkit - SPIGOT-7734: mutable
     private AdvancementTree tree = new AdvancementTree();
     private final HolderLookup.Provider registries;
 
@@ -40,7 +44,7 @@
             this.validate(minecraftkey, advancement);
             builder.put(minecraftkey, new AdvancementHolder(minecraftkey, advancement));
         });
-        this.advancements = builder.buildOrThrow();
+        this.advancements = new HashMap<>(builder.buildOrThrow()); // CraftBukkit - SPIGOT-7734: mutable
         AdvancementTree advancementtree = new AdvancementTree();
 
         advancementtree.addAll(this.advancements.values());