This release note from the latest build would seem to confirm the intentionality of the principle if not the actual moment-to-moment desired state of the button.
You should no longer be able to load from the pre-battle screen, but you should be able to load from the deck building screen (i.e. where you open chests, level up, etc). Is that the case?
Why? That's where you get hints of what to expect in the next battle, so why not let us load a new party? Nope. I can Store my current party, or Clear parties, but the only time the Retrieve button is active is immediately after Storing a party.
It's a technical limitation unfortunately. The party validity is only checked at the looting/level-up stage, so if you load an invalid party before starting battle, the client will let you do that but the server will kick you from the battle. That leads to bad things, like stuck battle rooms.
