You present an interesting situation and challenge. As a basis of my response, I am going to presume that your boss is willing to be supportive of your desire to be kept in the loop with this DR to the extent of asking if they have discussed the issue with you before taking their time to engage. If they haven't, the boss will refer them back to you.
I will also say that regardless of the number of DRs you have, of course it is important that you be in that loop as those above you expect you to know what your team is doing and thinking which reflects on your leadership.
I don't know that you have or haven’t discussed this in very clear terms with your report however, ultimately, this is necessary in resolving the problem and is what I consider the best way for you to address this issue. During such a discussion I would encourage you to simply get curious and be open for whatever response you get. It should reveal a lot. Questions to be asked include:
1. What are you wanting to achieve when you take your thoughts/ideas to others in the organization and do not include me?
2. What are you not getting or worried about not getting from me in bringing these things to me first?
3. Why might my being involved be important to what we are all working to achieve in the organization? (the answer will reveal what things need to be clarified in terms of your role and the importance of that)
4. What are the ways in which you would like to get support from me that you value and may not be receiving today?
As self-serving as this may appear, once you have had your discussion, this person is an ideal candidate to work with a coach. Through that relationship he/she can gain clarity around what it is he/she wants to achieve … in their position and in their career. Then they can explore and find a path to achieving this what doesn’t operate outside of the appropriate chain of command and/or alienate others along the way.
I applaud what you are out to achieve!