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!
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.
That’s a good point. I’ll drop it on my party again after my next long rest and see what happens.