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.
In actual 5E there isn't back stab, there is sneak attack. Rogues do not have to be behind or even hidden to get it. If they have advantage for any reason, or if another member of the party is within 5' of their target, they get it. This is just one more way the deviation from 5E rules is causing an issue in BG3.
To Seraphael a few posts above, BG3 does show hints of greatness, but not in the area of "faithfully" implementing 5E rules, like at all, which is what this whole thread is complaining about, and for many, thus far the most disappointing aspect of what is otherwise a pretty fine game. If they can fix that, imagine how quiet the forum would get. This is THE biggest problem I am seeing people, myself included, have (not of course the only one, but the biggest).