The problem with backstab is that an ennemy in front of you, engaged with you suddenly "don't see you" as soon as you jump behind him. Just jump as a bonus action at each turn and backstab your ennemies at each turns.
TB doesn't mean inconsistency because everything is frozen. As soon as you're engaged in any combats, you shouldn't be able to backstab anyone except if you succeed to hide (or eventually attack ANOTHER ennemy already engaged by another ally and whose attention is focus on another immediate threat... which won't happen that much in a game that obviously hate melee characters.....)
Apparently you have never played demon souls or dark souls series. You just killed people in real time with only backsteps in the fight behind them and turned into the back. So yes that is a completely normal kill art and just as permitted in the game. There is no need for further discussion.
What the hell are you talking about ? Dark Souls ? We're talking about Baldur's Gate and D&D...