It happened to me recently. I bought a week ago the Xiaomi Pad S6 Pro 12.4 , which is a 3K IPS panel, not OLED.
It has no flickering, with the Opple Light-master the measurements are healthy, but for some reason after about 20-30 minutes looking at the screen I started to get dizzy, wanted to vomit and had to lie down on the bed because I was going to faint.
The funny thing is that I had a matte and anti glare glass on, so that didn't solve anything. But I changed the screen refresh rate to 144hz, the colour mode to sRGB, turned on the blue light filter constantly and added texture, lowered the contrast, and I've been tweaking the font size and other stuff, and now I don't get dizzy, my eyes don't hurt or anything.
So I don't understand anything anymore, I don't know if the problem was the colour mode, the hz, or what.