If you truly want a mean, smart AI, then the enemy should only target my downed party members if it has a good (~75%) of killing them before I can Help them. Since a character can take 2-3 hits before dying (depending on their death saving throw), but it only takes a single action to bring them back up, the players is favored unless the enemy can hit my downed member multiple times before I can Help them.
However, a big underlying problem is death is so easy to fix. You can buy scrolls of revivify or use skelly-boy to revive your party members. Thus, whereas in PnP the party would always try to save a downed member (you're a bit of a jerk if you let a fellow player's character die when you could have saved them), in this game you can just say "eh, I'll resurrect them later" and focus on winning the fight.
p.s. If the BG3 rules were changed (to match 5e RAW) so that the Help action only stabilized downed characters, then the action economy would much more heavily favors attacking downed party members. It would also eliminate these whack-a-mole scenarios where a party member is downed, you Help them up, they're downed again, you Help them up again, and repeat...
