I appreciate the long reply, but to me that first image you replied to looks marginal, and the one below is slightly after the ball is played. Alexander-Arnold is moving towards goal, and Nunes is moving away, so a couple of frames earlier could easily put Nunes offside (especially as he's running out quickly to try to get back onside).
While you say everyone is in line, I can see what you mean, but it's not that simple. They are approximately in line, but the defender, the ball and the attacker are spread over nearly 40m, and the ball is yards to the left of the last defender. All three are moving in different directions. It's simply not possible for the human eye to track that accurately, hence why close offsides get called wrong all the time. Remember the movement from an offside position to an onside one, and the two headers, takes place in a fraction of a second.
It's a really tricky call for a human, and I bet if you went through all the VAR offside overturns so far this season, you'd find many similar decisions, which aren't being debated because VAR was working properly.