The Scale Guards attack only the first position, as though their text read “Deal X damage to the first enemy” rather than “Deal X damage to an enemy.”
Thanks to Snakes and Ladders event, Scale Guard defense teams are easily encountered, so this is pretty easy to verify, or you can create a defense team and attack yourself.
I really really don’t want to have to post 10+ screenshots of this happening ten times in a row. 
You are right, though i don’t think it is a bug, the Scale Guards just don’t give a damn.
I was just about to post this same thing.
Not exactly. From what I saw if you already have a poisoned troop he will attack it no matter where in the line-up. Otherwise yes, he attacks only the first spot. So it’s most likely not a bug on AI’s part, just
Yep what @GemsCollector said. I’ve noticed he attack my troop in 2nd spot. So it was first two SG killed my first troop, 3rd SG attack my 2nd troop, then I summoned a troop, 4th SG attack my 2nd troop again because it was poisoned already.
This just came up in my guild chat lol. I countered that they don’t always fire at the top troop, sometimes they make matches for no reason 
As others have said, not a bug, but definitely simpler AI than most troops. Prioritizes poison, then position.
Yeah, that happens all the time regardless of troop. Fully charged Mab can wipe player’s team? Better take some red gems.
And don’t get me started on AI’s use of Maw. Maw is full? Let’s eat Alchemist instead of something dangerous (Gard/Famine/Kraken/etc) that’s fully charged.
if you have a unit that can be 1-shotted the scale guard will target that instead of first enemy, but besides to that i agree xD
