First off, please use full URLs (you can embed them in text using the little icon that looks like a chain) when linking to MB entities; those MBIDs don’t link anywhere otherwise.
Second, I would venture a guess that you added the release without selecting a release group. When you add a release from anywhere other than an existing release group, there will be an empty text field where you can type/paste in the name of a release group and from there you can select the one you want. If you leave that field blank, it will automatically create a new release group.
BTW, the thing to do when you accidentally create a duplicate entity is merge it.