Reposting my analysis to a different similar comment as a response to your post.
Both of them would have had Foden in behind the United defender in the box and who is to say a chance couldn’t have come from either (though, one was obviously offside, but we know that doesn’t really matter now)? One was very tight, so why did the flag immediately go up?
As far as goal scoring opportunity playing a part, are linesman now doing xG calculations before deciding whether to raise their flag for offside? What is the xG threshold for immediate flag raise versus delayed flag raise? Who set the the threshold? Are they doing it in their heads or is someone confirming it in their ear the moment the potential offside is identified?
And, in the case of the United equaliser, it wasn’t tight, nor was it an immediate goal scoring opportunity (xG from his position when the ball was played forward is slightly higher than 0.00). Rashford was well off from the moment the pass was played, not even in our final third when he was offside, so why was the flag delayed until after Rashford had run 20 yards over the ball and Fernandes swept in to put it in the back of the net?
In fact, at no point was it a particularly great goal scoring opportunity,
except that all three of our players were defending a ghost offside player who had possession of the ball. Really, there is no goal scoring opportunity if Rashford is not allowed to run over the ball, as either Akanji or Eddie easily clear it before Fernandes arrives.
Fernandes even shoots from outside and almost dead centre of the box which is a relatively low xG position.
Foden’s position for both of his immediate offside calls would have much higher xG.
Nothing about the officiating during the match makes much sense outside of specific potential frameworks of utter incompetence or outright manipulation.