I got a new Lenovo T14 laptop at work to replace my trusty Lenovo X280, which causes zero eye strain.
Used the T14 with the same external screen that I have used the X280, for 4 hours and my eyes were bloodshot and extremely irritated. I could not even watch TV in the evening and now in the morning my eyes are still stinging and sore.
I use both laptops only with my HP ZR2740W external monitor.
So this proves beyond doubt that the source of the problem is the Laptop display driver or display adapter hardware.
Since the windows version is the same, It narrows it down to either display driver or display adapter hardware.
Since both are intel graphics, I would guess it is something that they are trying to do with the Temporal Dithering.
I cannot get ditherig to work, it gives "unable to load dll" so I cannot judge if Ditherig would make a difference.
I'm also sensitive to PWM, have been for 30 years, but this extreme sensitivity to, what is possibly caused by Temporal dithering, I have been sensitive only since last February.
X280 does not cause any discomfort either with it's own FHD screen, when used 100% brightness.