Offside goal in Ch League

[Nevadaref]

I'm watching the Man Utd v. Kyiv match from the Champions League right now and noticed that the AR unfortunately missed an offside infraction by Man Utd on the first goal in the 31st minute. Perhaps someone will post a video clip of it, but I will just describe the play because it is quite instructive.

Man U is awarded a free kick from about 45 yards out. All attacking players near the top of the penalty area are onside when the free kick is taken. However, one attacker runs forward faster than the other players and is in an offside position when the ball is headed by one of his teammates in the rush. The ball then strikes this offside player in the back of the head and crosses the goal area where it is headed into the net by another teammate.

It is certainly true that this player was onside at the free kick, but when his teammate headed the ball his position must be judged again, and at that time he was in an offside position. Therefore, when the ball struck him, the AR should have raised the flag and the infraction should have been penalized.