Xbox Series X shutting off during game play

It always runs hot even while watch Netflix or other format, it only shuts off during those games though now that you mention it. I will contact them thank you so much for your help.

Call of Duty: Cold War and it happened twice during Modern Warfare

Assassin's Creed achievements not popping is a known issue. So is the quick resume, that's got some issues too. For now, it's best to save and quit when you want to start something else.

What game are you playing when this happens?

Every report I've seen of this, it's happening during playing Cold War. Both games probably use the same game engine, so it could be causing the overheating issue. Other games don't seem to effect it. What I would do is turn off the console, let it cool,
then later try some other games, see if they have the same issue. If not, then you will know it's Cold War & Modern Warfare that's causing it. Once you're certain the games are what's causing the issue, then you should report the issue to Activision, Infinity
Ward & Treyarch. Hopefully they will be able to work with Microsoft to issue a patch which stops the console to overheat while playing the games.