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.
Wrong there is no character facing in tabletop 5e. That is the point of perception... it is believe you are ware of your surroundings and your character is looking all around them for the fight. It makes no sense to be staring at a wall during combat. That is why tabletop have never used it outside of an optional rule.
Point was that it is a flaw not to have facing.
It is equally a flaw that there is no reaction to a character obviously trying to get behind you, or trying to jump away or over you. Should warrant an attack of opportunity. Disengage is also nonsensical, with and without the tabletop versions action economy, where disengage would be an action in most cases, and a bonus action in few cases. The optional rule in this case, is sensible, another rule of having multiple combatants facing one should add to hit chance in my opinion.
The thing is. The whole combat round is 6 second, no matter the number of participants. And everything is supposed to be imagined happening at the same time. Turns just bring order in the chaos that would transpire if every player started shouting what he wants to do at the same time. Lets look at a duel example. In this case it makes no sense to keep facing the same way all the time, no matter your opponent actions.When you add other enemies that are engaged with you it could be reasonable. But for this case there is flanking rules. I get that in a video game it is not easy to always rotate a character like it is a fidget spinner. Nor will it look sensible.