There wasn't any need to push that high and vacate his zone. Bellerin didn't read correctly the situation.
He's followed Willian where he shouldn't but that image doesn't represent what happened in real time. I remember watching the game and saying that Mkhi hadn't tracked the runner. He barely jogs and doesn't even reach a sprint. Players get pulled out of position sometimes. It's up to others to cover. Mkhi's man is Alonso and he loses him.