OP I get your argument.
Example: a car. You are standing at the rear of the car. You are behind the car. You open the boot. You are in front of the boot.
By that logic, you can be at the rear of (behind a person) but be in front of their back or bum or whatever.
however that doesn't change your physical location to the rear of that entity. You are still behind them. 'In front of' as a phrase just means in the location of (usually facing towards) which is where you are confused.
Another example (to confuse the matter even more): take an object that has no discernable front or rear. A closed door. Person A is standing on one side of the door. From their own perspective they can be both in front of behind the door without changing their physical position. Person B is on the opposite side of the door. To them, A is behind the door and they are in front of it. Person C is on the same side as A. To them, A remains in front of the door. So A is simultaneously in front of and behind the door, depending on who you ask.
Take this back to a physical person, Person X. They have a discernable front and rear. Front being (obviously) their face side and rear being their back side.
Person Y stands facing X's face and Person Z stands to face X's back.
Person Y is in front of X's face AND in front of them as an entity.
Person Z is in front of X's back/arse/ankles but behind X as an entity.
Shitty diagram included.
Hope this makes sense, I'm so hungover.