The devs didn’t make it that way intentionally, and don’t believe it exists. Why it happens isn’t clear, and between what I can glean from the error messages we see, and the code they’ve posted or talked about, it’s likely that their code is too abstract to easily understand why it produces certain outcomes.
I really don’t think taking defense composition out of the hands of the players is a good idea, or an approach they’ll ever pursue. First, there are plenty of other modes where the enemy teams are determined by the developer – player-chosen defense teams are the defining element of PvP. And not only would it decrease player agency, as others have already argued, it would decrease player identity, at least in terms visible to other players. Already, there’s the complaint that player avatars are pointless because there are so few opportunities for other players to see them. A player’s defense team is one of their only opportunities to show what they’ve collected, show off their skill, or creativity, or sense of humor (or just whose gameplay videos they watch on YouTube).
If people are only playing PvP because of specific rewards, and not because they want to fight other players’ teams, then perhaps the developers should provide alternate game modes for those rewards. But gutting what little element of PvP there is in the PvP mode doesn’t make any sense.