I don't see a problem here but I also don't play DnD. Of course the saving throw has to be made for each target affected as each target has it's own premises to pass the throw. And why should there be a unified damage roll for all entities affected? The game should roll for damage for each target as each target can be affected (burned) in a different way. I don't get it why it is important that it is a single spell or several spells which hit different targets.
It's the same when you cleave at several targets with a sword. It's the same sword but it can hit targets for different damage. If you refer to reality (ok, often a bad idea ...), the explosion of one grenade can affect targets quite differently even if they are close together, one dies, another has much less injuries for example, depending where they are hit in what manner.
