This is not a bug, this is a well-known feature that works correctly. In the past there could be an unlimited number (or a larger number than two, anyway) of rivals for one team. It was limited to two to make it more fair but to also allow a little bit extra excitement to this feature.
In summary, not a bug and therefore the thread is closed. If you feel you must, please suggest a change in the suggestions forum (I propose the minor suggestion thread).