Problem with Duty forced into Expansion Content...

[Niwashi]

Unless I'm missing something here, the game doesn't care about types of classes, just their role

Sort of. It will try to mix both melee and ranged damage dealers into each party if they're available, but can suspend that rule when it needs to, so will sometimes make a party where all DPS are melee or all are ranged if there simply aren't any of the other available. (For this preference, it's looking at the range of their attacks, so Bards and Machinists get lumped together with casters rather than with the other DoW damage dealers.)