He might be doing exactly what his coaches are asking him to do, I don't know, but imo it's one of those defensive details he needs to be better at. That's not an unusual defensive situation for Mateychuk where he isn't very assertive or sacrificing in terms of shot and pass blocking. Since his draft year, I've rarely seen him go down on his knee and land his stick sideways in attempt to take passing and shooting lanes away. I think he could've done that and got a little closer to Marques to prevent the goal.
Maybe those kinds of "bad habits" are typical for rover type defensemen because they're always looking to turn play to the other direction, even when a man down. It almost seems like he's trying to bait Marques into making a poorly timed pass to #18 in hopes to create a counter attack...?
I don't think he made the right decision but I'd be curious to hear how others see it, especially those who are optimistic about Mateychuk's chances of making the team out of camp next year.