Right, they're a good team they're going to break the tactic down a few times. The whole point of the narrow formation was to concede space down the flanks to encourage Brighton to play the ball there, then seal off the pitch vertically and press them against the touch line. I made some images:
Here's how they lined up when Brighton started. Brighton typically try to bait a press from this position, opening up space in the middle of the pitch and then exploiting that for quick attacking moves. United instead went narrow and blocked the middle of the pitch. Rashford and Hojlund made curved runs toward the centre backs to push them to move the ball down the flank. I'm not totally sure where Brighton's central forward players were because the camera didn't really show them that much, so I'm mostly guessing here. Onana also wasn't this high up the pitch, so you can ignore that bit. The diagram represents the shape, not the exact area of the pitch this usually happened.
(Forgive me, I made this image first and only figured out how to change the keeper's colours afterwards, and I'm too lazy to go back and redo it)
Once the ball was played into the wide area, United would attack the ball in a press like this, sealing the pitch off to one side in a vertical line. This worked pretty successfully and United stole the ball from them pretty frequently, especially in the first 70 minutes of the match. The goals occurred when this press failed, at which point the midfielders needed to track runners back, which they didn't do.
BTW this is not a particularly outside the box tactic, lots of teams do a mid/high press like this from narrow formations. IIRC Rangnick used this kind of 4-3-3/4-1-2-1-2 formation sometimes. Every single tactic is going to leave weak spots for the opponent to exploit, that's just the nature of football, and that's where the importance of work rate and situational awareness comes in (i.e., you need to track the fecking midfield runners). The opponent breaking your tactic a few times is just part and parcel of football and something you have to be able to deal with to win matches.