Well, what "should have been" isn't what's in the laws.
Neither is you can't tackle from behind. It was in the laws 20 years ago, as a straight red, but not for long.
you can tackle from behind if you win the ball only ? but if you catch the player and he goes down then its a foul and a yellow card if it was like on foden last night > simple the VAR team got it so wrong ? the referee got it wrong in not seeing the foul in the first place and waving play on