Il also keep it in spoiler tag. Do want to reply but not create a massive wall o text.

Like I said it is a very literal interpretation. Things like facing generally only work if you are using a battle grid though so it makes sense that they rules for people who dont use one as well.

While I might be looking a little to much into it, it clearly says a person that cant see you gives you benefits for attacking them. People dont have eyes in the back of their head so by this train of thought someone attacking you from behind you, you couldnt see. That said, the section where it says this also mentions beeing hidden or invisible so it could be that it was intended to just factor in those conditions. I personally dont grant anyone attacking someone from a flank advantage when im DMing because their enemy might not see them. It completly breaks combat and we clearly saw that in bg3 as well. When I rule something in my games the rules apply to (and for) the monsters as well and I dont want to TPK them... But I do get where it probably came from. Mind you, I think the DOS series also have a flanking/backstabbing system so it could also be a leftover from their previous games. Im not sure which scenario is true.

Something to mention in favour of people needing invis or hidden status for this rule is things like attack of opportunity for example. It requires you to be actually able to see your target. Now if theyre moving behind you and they leave your melee range you still get to do an attack of opp though. Because what triggers it doesent requires your target to be infront of you. Which by itself would also be in favour of people beeing able to see something behind them so long as theyre aware of it. I think only invisibility (and disengage) would let someone move out of melee range without triggering an attack of opportunity. Or teleportation.

So I agree with you that the implementation doesent make sense. Just trying to explain that what Larian had in place was in the phb. Albeit a very literal interpretation of some rules that might have been extended to places it wasent intended. Or a leftover system form their previous DOS games.