Not always true: for example in my current playthrough I had to choose between gaining advantage on attack with Astarion (and his sneak attack damage provides a big reason for doing so) and being exposed to subsequent attacks. I choose to attack anyway and Astarion got downed in no time. Positioning yourself behind an opponent is not always risk free, and with better AI setting up traps this could be a very real problem for those who favor high risk/high reward approaches.
Edit: same is true for climbing up high. Sometimes the AI follows you and then throws you down causing falling damage and destroying your positioning advantage (this happened to me with the Kuo-Toa encounter for example).
I don't see the relevance of your anecdotes. If you had an ally beside the enemy, Astarion would be just as exposed and still get the Sneak Attack, but it would be by the rules. By the rules a Fighter cannot get advantage fighting an enemy 1 on 1 and going behind them, but they can in BG 3.
No one ever said that enemies could not attack you on high ground.
