There is GM leeway allowing for giving Advantage/Disadvantage for circumstances that they think fits.
Also, specifically, there are optional Facing rules on page 252 on the DMG that give advantage for attacking from the rear.
The optional Facing rules says u can use your reaction to turn around and avoid being attack with advantage and this is not implemented, if someone walk behind of you, there's no shit you can do about it. And I also never played a TTRPG where facing has been used. The free advantage you get here has no downside, no resource cost. I think this should be changed if not removed.
If front/back has to be here, it shall at least trigger AoO, or allow the one being backstab to turn around as reaction, not just free advantage here.
