Please watch the attached video all the way through since I believe that it does eventually reveal the cause of this issue.

I would like to add a note following the end of the video, the inspector reveals that the chunk association is valid (as does Debug.Log #1), yet Debug.Log #2 also reveals that the chunk instance compares to "null". This means that the problem may be located within your custom overload of the '==' and '!=' operators.