Larian reversing their backstab/flanking advantage homebrew could be indicative of them working on the Barbarian and arriving at a very late epiphany that they were screwing Barbarians and their Reckless Attack over with it. On the other hand, it could also be that Larian simply realized their homebrew promoted shitty gameplay, but given their fondness for adding way too many highly questionable homebrew additions - the former is more likely.
It wasent homebrew. It was a literal interpretation of the rules. If you attack someone who cant see you (for example if you are behind them) you have advantage.
Standing behind someone doesn't mean they can't see you. PHB pg 177: "In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you" and "you can't hide from a creature that can see you."
The DMG has optional rules for Facing, further clarifying that according to base rules a creature is assumed to be constantly looking in every direction. And Larian's implementation didn't include all of the other components of this optional rule, such as using a reaction to change the direction you're facing.
RAW, even standing behind total cover doesn't grant you advantage on your next attack. You have to specifically take the Hide action in order to be hidden, and only a successful Stealth check against the enemy's passive perception will you get advantage on your attack.
Edit: The last sentence ^ is incorrect. While standing behind total cover doesn't grant advantage (because peeking out makes you visible), you don't need the Hide action to get advantage. Just to be unseen.
