Fix invalid model cache on mission change

project/code/TMV/ModelLoader.cs

@@ -34,7 +34,7 @@ public class ModelLoader
         // We don't care if this is null actually, we'll still cache that it's null lol
         var model = Timing.TimeStage("Load Models", () => { return LoadModel(_pathManager, ref campaignName, modelName); });
         _cache[(campaignName, modelName)] = model;
-        return model;
+        return model?.Duplicate() as MeshInstance3D;
     }
 
     public static MeshInstance3D LoadModel(ResourcePathManager pathManager, ref string campaignName, string modelName)