Close your model tree and open it up again. Sometimes the model tree doesn't update properly. I've seen this behavior also when adding feature parameters to the model tree. Either way, you can still only have one model in the tree at a time.
This post made me curious, and it turns out I have the same problem, even if I change models and re-open the model tree. Though I don't consider it a real problem, it is puzzling. I've never noticed it before on any other version.