Ok, I found kind of a discrepancy. Not a bug or anything though.
When there's at least one party member in reserve, the dark rectangle is permanent, before, during and after using the party management.
However, when you have all your reserve slots empty, there's no dark rectangle before you use this function, and after you use it, it's not disposed, it stays there until you select a new scene.
Doesn't it make more sense that if there's not a rectangle before, there shouldn't be one after?