Just watched it again on City+, the commentary for the penalty is as follows, from 8:12:
" Ramsdale's spilt it, De Bruyne's brought down, penalty!.. no, what's Michael Oliver given? What's he given Michael Oliver?... Well it looked like a foul on De Bruyne, what's he given Michael Oliver?...He's given a free kick to Arsenal and not a penalty to City. Well I am not sure what he thinks has just happened there? Andy?... It's looks as though he's insinuating De Bruyne has... stepped across..., yeh... I can see Partey has just got his foot in there, and De Bruyne's has...yes I can see why, but initially, yeh, I can see it's the right decision, and in fact it's a very, very good decision because you can see the initial thought is penalty, and I think everyone of us thought it was, but, looking at it, he's correct...Well in reality what happened was De Bruyne went to hit the ball into the back of the net, after Ramsdale's spilt it, Partey came in to stop De Bruyne, De Bruyne's took a swing to where he thought the ball was and kicked Partey, and the two of them then fell over. So it is one of those where Michael Oliver has the gut instinct that it was the attacker that caused the collision, rather than the defensive midfielder, and it's the reason Andy felt it wasn't a penalty, "
I still think Oliver was very biased to Arsenal, but unfortunately I have to agree with the City commentary on the early penalty.