While facing is optional rules, attacking someone who cant see you already grants advantage. Unless the dm rules that for ease of play everyone just has 360 vieuw.
After all in combat its not uncommon for people to move around so just translating that to 360 degrees vieuw has some merits. That said I generally dont want to punish my players when they try to do a tactical move and hitting someone in the rear when they csnt see you would fall under that as well.
I think ingame its not really a problem though, the AI makes use of it as well after all.
I never see AI attack me from my back, unless I have my back towards them, I doubt AI is smart enough to do this.
