Yeah, there was a lot of dysfunction happening out right with Pépé and Bellerin earlier this year. And that was having impacts all over the field.
First, on the right Pépé and Bellerin were not connecting. Bellerin was staying back and Pépé was coming back to get the ball, ending up creating alone in traffic without the benefit of movement and space that makes him dangerous.
Second, because that wing was clogged, Aubameyang also became ineffective in 2 ways. It was easy for teams to just put their best defenders like Kyle Walker on him and also cut the supply chain down the left. But it also meant there were no diagonal balls for him to feed on from the right. This is what Liverpool are great at: balls from TAA to Mané and Robertson to Salah.
Against United all this got unlocked. Bellerin got higher. Willian kept feeding him great balls (including the penalty) and they both kept finding Aubameyang.