Skeleton Sorcerer slowed ally

Since the Slow spell only affects targets of the caster's choice, I'm surprised that a Skeleton Sorcerer somehow affected a Skeleton Knight along with my party. I saw this happen in The Arena Round 2. I expect Slow only to target enemy creatures.

I hope it's an NPC-only problem and that Slow is safe for my characters to cast in a volume targeting a mixed group of allies and enemies. If this does only affect NPCs only, then it's not a high priority, since it's a bug in the player's favor.