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.
The issue here is that those seemingly small changes can have unintended effects on other features.
There are abilities, that increase the damage of a single roll. So if you roll once and apply the damage to all targets, the bonus damage affects all targets. If you roll individually, the bonus damage only affects one target, making those abilities noticably weaker.
Of course larian can do whatever they want, but I highly doubt their rule variations have a positive impact on class balance.
