There is a backstab as in: "Ohohohoho, I am sneaky boi and can attack this guy without him having any knowledge of my presence at all!"
And then there is backstab as in: "Oh noes, I have no other ways but to dodge in order to defend myself from this guy who just jumped behind me, clearly he have a much harder chance of hitting me now, when standing behind me, where I can not really block or parry his attack, but only dodge, as opposed to when he is in front of me when I have greater variety of defense mechanisms."
At the end of the day, this is a general flaw in D&Ds turn-based systems, rather than BG3 itself.
