All this visual Hell started 7 years ago with the iPhone X: The first OLED iPhone. That’s when this thread of doom started.
So the problem is likely related to one, some, or a combination of the hallmarks of OLED displays.
1. Pentile sub pixel arrangement
2. PWM flickering
3. Temporal dithering
In regards to PenTile sub pixel arrangement of the OLED display, the DPI of the screen is actually much lower than the specification states due to the way the pixels are arranged.
OLED iPhone specs state the resolution is 460 dpi, but in real world use when actually looking at the screen, the resolution is a lot lower.
This “graininess” of OLEDS could be a factor in eyestrain as well.