PaperMC/nms-patches/DataConverterMap.patch

--- a/net/minecraft/server/DataConverterMap.java
+++ b/net/minecraft/server/DataConverterMap.java
@@ -31,7 +31,7 @@
                 Typed<?> typed1 = typed.getOrCreateTyped(opticfinder1);
                 Dynamic<?> dynamic1 = (Dynamic) typed1.get(DSL.remainderFinder());
 
-                dynamic1 = dynamic1.set("map", dynamic1.createInt(dynamic.getInt("Damage")));
+                if (!dynamic1.get("map").isPresent()) dynamic1 = dynamic1.set("map", dynamic1.createInt(dynamic.getInt("Damage"))); // CraftBukkit
                 return typed.set(opticfinder1, typed1.set(DSL.remainderFinder(), dynamic1));
             } else {
                 return typed;