Cool. Done. The advantage rules state, "The GM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result."
The GM is Larian. They have decided. Rule followed.
Swen Vincke, is that you? Oh wait, you're just some random poster without the position to say Larian has decided.
They have not decided. I say that because that's the reason why Larian put the game out into Early Access. If they have firmly decided and have no interest or intention of changing anything, then there's little point in the hassle having to support and update Early Access. They could just have easily kept the entire game under wraps until the full release and said "this is the game, we're not interested in what you think, deal with it".
I have evidence that a decision was made. It's been programmed into the game. It didn't happen by accident. Someone didn't trip and their face hit a keyboard and tons of code went into the game. First a person had to decide to do a thing, then that thing was done. That's typically the order of operations for human endeavour.
Now, your argument is that they might change their mind. Of course they might! But at the moment, they have made a certain decision, and put it into the game. They could make another decision later, and then change the game. But the current "ruling on the table", in D&D terms, is that high ground and backstab give advantage. Until the DM changes their mind, that's the governing rule.