Funnily enough, just this week after the latest Modern Warfare update my R13 with a 3090, doing the same, hard reboot, but only with Modern Warfare. 

The wifes R10 with a 3080 on windows 11 had the same issue you describe above but with all games in general, we sorted it by switching back to a Bios from late 2021. 

I will let you know when I figure out my issue incase it helps you, although I'm confident its not hardware related my end as I just throw cinebench r23 multi,  time spy and Heaven Unigine on a loop and let them run with zero problems. 

Liquid or air cooled?

What CPU, PSU & RAM in that rig?

Sorry, I should have known to list that.

AMD Ryzen 9 5900X Processor (12 Core, Up to 4.80GHz, 70MB Cache, 105W)
32GB (2X16GB) 3200MHz DDR4 XMP UDIMM Non-ECC
Alienware Chassis with 1000W Power Supply (Dark Side of the Moon)

Liquid cooled CPU. Air cooled everything else. Alienware Command Center show CPU and GPU temps never get into a dangerous level. GPU stays between 60 - 70 in heavy use. The system will often crash in the lobby before the gameplay starts. It will also crash when changing any GPU settings in the Alienware Command Center with no games running. 

We've done some more troubleshooting. 

• Uninstalled Alienware Control Panel and the problem persists.
• Drastically reduced the quality settings in the game and the problem persists.
• Reseated the GPU and power connectors and the problem persists.

Another issue I have identified is that the computer only reboots about 1 in 3 attempts. At first I thought my son wasn't giving enough time after a cold reboot but I tried it myself and we have to hit the power button a few times.

Here's what happens when the reboot fails -

1. The lights come on, the fans start up
2. The fans quickly slow down and become almost silent
3. The lights on the front of the case remain on
4. Nothing appears on the screen

Sometimes the keyboard will light up but then go dark.

My son is on the phone with Dell again to try and sort this out.

That is a nice rig . . . should not be crashing. Try uninstalling Dell Support Assist.

Consider an nVidia driver clean install;

Tube Vid instructional 

Display Driver Uninstaller 

nVidia driver download page 

Drivers were already uninstalled with DDU and reinstalled. The Dell technician did his best but the reboot issue is progressively getting worse. We've had to pull the power plug twice and hold the power button down 40 seconds to do some sort of reset just to boot. 

Good news is that Dell is sending a box and it will go back to them to repair. I just had this done to my Alienware laptop and they turned it around in 1 day. I will post back when I have more info.

make sure you're not plugging all power cords into one power splitter.

Well bizarrely for me it was Steam crashing my PC, all sorted now, hope you get fixed soon