I can answer this, and it’s working as intended, albeit not clearly. Scatter damage used to only deal enough damage to kill the troops that it hit, but with the introduction of the spell armour traits, this would always leave those troops alive because the base damage it was reducing was only equal to its remaining life, instead of the full damage. This was originally so that if one troop died, then the rest of the damage would still be dealt to the rest of the team, and there was no need for the excess damage.
Now, Scatter damage will do that same calculation, but the excess damage will be dealt randomly among all the targeted troops. It won’t prioritise the damage to a troop with a spell armour trait.
As we only display the damage actually dealt, this is something we could improve on and communicate better as you’re playing.