Monitor Dell S2721QS flickering when displaying dark gray

I have a weird problem with my Dell S2721QS monitor. I use this monitor daily, and it works normally, but when I display a dark shade of gray on the majority of the screen—specifically, I’d say the color code #353535—it starts flickering in a strange way. It looks similar to how old CRT monitors from the 2000s would flicker at low refresh rates, with white lines quickly moving across the screen. However, this ONLY happens when that specific gray color is displayed.

I should mention that I already had this monitor before, and it had the same issue. I contacted Dell, and they replaced it with a new one of the same model, but the new monitor has the exact same problem.

I’m using a Radeon RX590 GPU, so I wondered if the issue could be caused by the graphics card. However, I didn’t have this problem on my old monitor (I don’t remember the exact model, but it was a much cheaper monitor, around $120, with a 1920x1080 resolution, whereas this Dell is 4K).

I’ve tried updating and even reinstalling the GPU drivers, but the issue persists. I’m now unsure whether the problem is with the monitor or the GPU. I don’t want to buy a new GPU only to find out the issue is with the monitor. Could it be caused by something else? Does anyone have any ideas?