Mesh.CombineMeshes does not work when mergeSubMeshes=false

In my script, I am trying to combine all objects that share the same material color into individual submeshes on a merged GameObject. So far, I'm getting extremely frustrated with the Mesh.CombineMeshes "mergeSubMeshes" flag.

It appears that when I set this flag to "false," I end up with only one visible submesh, and the inspector for the GameObject says, "This renderer has more materials that the Mesh has submeshes." I'm grateful for any help!

 private Dictionary<Color32, Material> materialsByColor = new Dictionary<Color32, Material>();
 
 GameObject CombineMeshes (List<GameObject> gameObjects){
     var numGameObjects = gameObjects.Count;
     var combinedGameObject = (GameObject)GameObject.Instantiate(combinedMesh);
     var combinedMeshFilter = combinedGameObject.GetComponent<MeshFilter>();
     var combineInstances = new CombineInstance[numGameObjects];
     var colors = new List<Color32>(materialsByColor.Keys);

     GameObject obj;
     CombineInstance combineInstance;     
     MeshFilter objMeshFilter;
     int submeshIndex;
     
     for(int i = 0; i < numGameObjects; i++) {
         obj = gameObjects[i];
         combineInstance = combineInstances[i];
         objMeshFilter = obj.GetComponent<MeshFilter>();
         
         combineInstance.mesh = objMeshFilter.sharedMesh;
         combineInstance.transform = objMeshFilter.transform.localToWorldMatrix;

         // I want to put each object in a submesh with objects of the same material
         submeshIndex = colors.IndexOf(obj.GetComponent<Renderer>().material.color);
         
         // Prints exactly what I expect.
         print (submeshIndex);
         
         combineInstance.subMeshIndex = submeshIndex;
     }
     
     combinedMeshFilter.mesh = new Mesh();
     combinedMeshFilter.mesh.CombineMeshes(combineInstances, false, true);
     combinedGameObject.GetComponent<Renderer>().materials = new List<Material>(materialsByColor.Values).ToArray();
     
     // Prints an unreasonably low number, and it appears only one submesh is rendering
     print(combinedMeshFilter.sharedMesh.subMeshCount);
     
     return combinedGameObject;
 }