I'm pretty sure you can't sneak in D&D while standing behind an enemy.
You can, and can even hide in plain sight once good enough.
Fun > realism
Yes you see stealth during combat is completely unrealistic and is never done in real life.
Right Guerilla fighters? Right Snipers?
Stealth during combat isn't unrealistic... Just going "I am sneaking" while being directly observed without anything intervening is unrealistic.
As for "Sneaking while behind someone", not only can you... But realistically... you also can. You are clearly there, you are just being extra sneaky so the opponent cannot lay a beat on you as easily.
Stealth and "Can be caught" are not mutually exclusive. As well "Stealth" and "Know you are there" are not mutually exclusive.
