PSA: Sculpt spells will not save your party from spells that still do damage on spell save.
I almost caused a TPK in a big fight right now by dropping ice storm on top of my team. Everyone still took massive damage, and it killed Shadowheart's divine guardian which she had just casted. Read your spells carefully before assuming it's safe to cast them on top of your party!
Sounds like you ran into a bug somehow, because casting Evocation spells around your party is exactly what Sculpt Spells does (the whole point of taking Evocation spec is to be able to freely lob Fireballs into the melee). Or maybe you triggered a secondary effect since it would take a very specific scenario (plus a very high damage roll) to be able to one-shot a Guardian of Faith with Ice Storm. Even something like Wall of Fire only does the environmental damage if an evoker paints it over a party member.
Ice Storm is a little weird in that it will ignore Sculpt on a party member if you accidentally click on them, but it shouldn't hit anyone else in the party even then.
So does Fireball and yet that one does no friendly fire damage. Either Ice Storm is bugged or it's as the other commenter pointed out, Guardian of Faith detected a hostile action and triggered an attack on all nearby hostiles (your party), which dealt enough damage to it to kill it.