I beleive this is what is confusing people
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He is clearly in an offside postion here and at an advantage if the ball is played back into him. as he is yards ahead of any defender and the goalie would have to chose to cover him or fernandes running in postionally.
You will see, I would say 80% of the time a flag and the ref do the arm wave to say he ran onside from an offside position.
It is all down to the interpretation of the officials when the passage of play began, if it is when rashford puts it in as per the still above he is off, if it is from when fernandes crosses it is onside, the ref and linesman (and VAR) decided the first ball in wasn,'t relevent, but we have seen plenty given off like this and plenty given on more so at the swamp