S3221QS, backlight flicker, #5

I have had the S3221QS for about a month now and I've noticed it has a sporadic backlight flicker. I think (but am not positive) that it's happened since switching to a new graphics card about three weeks ago. I'm a photo-sensitive migraine sufferer and also get sim sickness (like motion sickness) from certain flickering lights.

Probably the first two weeks of using the monitor I had no issues, but during the third week I started suffering sim sickness and migraines while playing Diablo3. A game I'd played for years with no previous ill effects. I realized that at random times D3 was flickering (or rather the backlight was). Initially I spent a lot of time trying to troubleshoot the game/AMD driver settings. But the flickering was sporadic. I initially thought turning freesync off fixed the flickering, but it came back.

In the last few days I've been getting the same symptoms in Windows while using a browser or a spreadsheet (but the flickering was not visible). Finally today I noticed pronounced, sporadic flickering while using Firefox. It's most noticeable with a dark background. I've attached a link to a video of what it looks like. Video is 24 seconds and about 60MB. The video here shows why this had been hard to detect, there is flicker to start, but halfway through it just stops flickering. 

I believe the flicker is getting worse as it was initially not visible in Windows, but now it is. I think the amount of times it flickers during a day is getting more frequent as well. Sometimes the screen flickers for a few seconds, sometimes 30 seconds, sometimes longer. And there are periods where there is no flickering that could last as long as several hours. I think snapping a window to the edge of the screen triggers flickering. Or possibly it's when the mouse pointer transitions from the Dell to the Samsung.

I run dual monitor at the moment with the second monitor being a Samsung UD590 (28" 4k monitor) that runs at 60Hz as well. The Samsung has no flickering at any point.

I've powered the monitor off/on. Disconnected and reconnected it. Reset it. Switched it from the first to second displayport (I don't have a HDMI cable to test). Tried it as the only monitor and in dual setup. I haven't been able to test it on a different computer or with a different video card at this point. However, roughly the first week was using a different video card and I didn't notice any flicker then. Though I didn't notice flicker in the second week while using the new video card either.

I ran the self test diagnostics but there is no visible flickering to my eye during them*. I am however, not sure this is a foolproof test as the flicker is sporadic. I took videos of each test (each about 20seconds and 60MB) and there is some weird flickering in them. But I'm not sure whether that's the camera rather than the monitor.

* I have been able to get the screen to consistently flicker when snapping a window to the side of the screen. With a dark background in the window I can see the flicker reliably. With a solid green, blue, or red I can't see the flicker but I suspect it's still there I just can't consciously detect it.

Green

Blue

Red

Grey

White

The PC is custom built, Asus Crosshair VII Hero WiFi (x470) bios 4301, Ryzen 7 5800x, 64GB (2x32GB) Kingston HyperX Fury 3200 DDR4, ASRock AMD Radeon RX 6900 XT Phantom Gaming D (3 DP 1.4, 1 HDMI 2.1). It's running Windows 10 64-bit. All drivers and bios are up to date. I've seen the flicker on both 21.6.1 and 21.5.2. Original video card was a GTX 1070 connected via DP.

It's connected AMD Radeon RX 6900 XT DP out -> DP cable -> S3221QS DP in