dnd 5e – Draconic Sorcerer Elemental Affinity with ongoing effects

The Draconic Bloodline Sorcerer has the following feature:

Elemental Affinity
Starting at 6th level, when you Cast a Spell that deals damage of the type associated with your Draconic ancestry, you can add your Charisma modifier to one damage roll of that spell. At the same time, you can spend 1 sorcery point to gain Resistance to that damage type for 1 hour.

This reads that it would only apply to one damage roll of a spell, meaning that spells such as Immolation and Investiture of Flame would only benefit from the first damage roll of that spell.

Is this the correct assumption? If so, would it be too strong to rule that it applies on every separate instance of damage?