Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

Reported by another player, but:

(1) With Greater Rainbow Blast and Chain Lightning, DC was 29 on Stormbolt.
(2) Epic Call Lightning was taken and Greater Rainbow Blast was removed from the prepared spells.
(3) Stormbolt was cast at a DC of 27, 2 less - rather than the expected +2 to DC (going from 8th Greater Rainbow Blast to 10th Epic Call Lightning). The player also reported it did slightly less damage.

This means that Stormbolt is likely using Chain Lightning instead of Epic Call Lightning, meaning it is not properly registering Epic Call Lightning?

I can confirm this, my pc's only lightning spells are Storm of Vengeance and Epic Call Lightning. Consuming all Storm of Vengeance uses results in Stormbolt not firing at all, meaning it doesn't recognize Epic Call Lightning as a valid spell.