My AW3423DW has the same symptoms.

https://www.youtube.com/watch?v=7kp0NMrOGkg

TS Steps:

Shut down the PC so that there is no signal input.
- Press and hold left on Joystick until grey screen appears
- Press right on joystick to cycle through each color

You need to contact Dell technical support.

I purchased it back on March 21, finally got it Jun 23. (Firmware: M0B101) Only used for 3 days and had flickering on 8 bit 175hz every 1 min. 10 bit at 144hz was ever 30-45 secs,  even in the diagnostic mode where it shows all the RGB colors each screen color had flickers .. Long story short  only had the monitor less then 7 days and the were going to send me out a refurbished one, that will take 1-2 months to get, Asked if my warranty is paused and or be extended cause of the wait time, they said "NO" . Also i told them you sending me out a refurbished, if that one has problems since it be over a 1 month wait can i get a refund, they also said "NO" So for having it for less then 7 days. I told them to refund it not going to get a refurbished one for that price. with in a 7 day time span of having it. 

Here is a reddit thread about this issue.

https://www.reddit.com/r/ultrawidemasterrace/comments/tbtich/aw3423dw_flickeringbrightness_adjustment_issues/ 

Some folks have success by doing different things.  I'm trying them to see if it's something that will go away before I decide to return it.  There is no way I'm going to put up with regular flickering for the price of this monitor, despite it having the best looking picture on a monitor I've ever seen.

Update: Sept 19-2022 ..   After I returned the first AW3423DW with firmware M0B101, and flickering issues back on jun 29-2022. I re-ordered the monitor again (Back in july) hoping to it may be just a small defect on the one i previously had.  The new on came in with M0B102 firmware, and so far everything is working as it should.

- 8Bit color @175Hz ----> no flicker with and without gsync on 

-10Bit color @144Hz ----> no flicker with and without gsync on

-12Bit color @120Hz ----> no flicker with and without gsync on

Hope everyone that order get no problems with their monitors, thanks Happykitty 

Just wanted everyone to know that I returned my flickering monitor for a replacement and it was a fairly pain-free experience.  Got the replacement in less than a week, and it works great.  Then I returned my flickering monitor to Dell to close out the exchange.  I've been using my replacement monitor now for at least a couple of weeks and it has zero issues.  I love the picture and the HDR experience is really nice in all games, and amazing in games that were designed to work with HDR.

I had the same problem as described here. I also called support and got a replacement because I was not able to start the diagnostic. When I got my replacement monitor most of the flickering was gone. I still had problems in a webpage and some videos. After trying a lot of combinations I found the solution that worked for me. I disabled the Desktop Window Manager overlay by going to the following registry location. HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\DWM and adding a dword called OverlayTestMode and giving it a value of 5. After a reboot the screen was flicker free. This fix is originally for windows 10 but my OS is Windows 11. Hope someone finds this useful.