2024 Elemental Affinity Question

Elemental Affinity is the 6th-level feature for Draconic Sorcerers. From the 2024 PHB:

"Your draconic magic has an affinity with a damage type associated with dragons. Choose one of those types: Acid, Cold, Fire, Lightning, or Poison.

You have Resistance to that damage type, and when you cast a spell that deals damage of that type, you can add your Charisma modifier to one damage roll of that spell."

It seems obvious that Fire affinity only adds damage to the initial casting of Wall of Fire, and not any damage taken by creatures standing in it on subsequent turns. 

My question is regarding Lightning affinity and Witch Bolt. It's clear that this feature adds your charisma modifier to the damage of the initial casting of witch bolt. Does it also get added to the witch bolt bonus action damage on subsequent turns?

The more I read the description, I'm starting to think the answer is no. Since the bonus action is applied via maintaining concentration on the initial casting, I don't think it counts as casting another spell. However, if you miss on the initial attack, would it then get added to the first bonus action you use?

I believe your interpretation is correct.  You get the bonus on one damage roll of the spell.  Whether that's on the first turn because you hit, or the second because you missed, it's still one roll per casting of that spell.

Quote from wagnarokkr >>

In addition, I think since it doesn't say otherwise, you can choose which damage roll it applies to. There are probably a few narrow cases where it might matter, such as spells like Flame Strike that have multiple damage rolls with different types where a target might be resistant to one of them and not the other.

 That's an interesting read.  I'd agree with you RAW, but I'd argue RAI is that it should only apply to the roll that uses the damage type of your affinity.

Certainly fair enough!