Apparently TransferFunction2D was a MonoBehaviour (not on purpose)

Assets/Editor/TransferFunction2DEditorWindow.cs

@@ -135,7 +135,10 @@ namespace UnityVolumeRendering
                 {
                     TransferFunction2D newTF = TransferFunctionDatabase.LoadTransferFunction2D(filepath);
                     if(newTF != null)
+                    {
                         volRendObject.transferFunction2D = tf2d = newTF;
+                        needsRegenTexture = true;
+                    }
                 }
             }
 

Assets/Scripts/TransferFunction/TransferFunction2D.cs

@@ -4,7 +4,7 @@ using UnityEngine;
 
 namespace UnityVolumeRendering
 {
-    public class TransferFunction2D : MonoBehaviour
+    public class TransferFunction2D
     {
         [System.Serializable]
         public struct TF2DBox