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.....)
Backstab contradicts the game world. These characters aren't standing in place, facing one way. Your character is moving around in combat, swinging, dodging, etc... That's why you threaten the space "behind" you.