Interesting thread- I thought I was crazy, I've had the same issues! I had a 6S, then a 7, then an 8Plus with no issues. Tried an XS last year and quickly realized headaches and eye pain were being caused by PWM (or I thought it was anyway) so I returned it to get the 8Plus I had for the last year.
However now with my 11- not pro- I am feeling some minor pain in my left eye, my eyes feel warm and dry when looking at the phone for a few minutes, and when I scroll it makes me feel a bit odd. That part may be the size of the screen since I've never had a phone this big before. But not sure why the other symptoms?
I kept thinking it was improving so I didn't return during my 14 day return window with my carrier-and the symptoms aren't terrible, just annoying- and it makes me feel frustrated that I could use my 8Plus all day with no issues, but this after just a few minutes causes my eye discomfort.
I also turned off FaceID and the awareness features, that didn't help. I keep brightness way, way down (probably less than 10%) because I do definitely have eyes that are really sensitive to brightness- it's why I have to keep my TV and laptop screens way down, too.
But anyway, with all of that I haven't resolved it. The one more thing I will try is to order a privacy screen protector- some of them are also a bit shaded like sunglasses when looking straight-on, so maybe that'll help even out some of the contrast or sharpness that somehow could be bothering my eyes?
If that doesn't help.... I guess I'm kind of stuck for now because I'm well beyond the return window. I got a deal with my 11 where I was entitled to a $250 cash back via a Visa gift card 90 days after activating the phone due to joining a new carrier.... so I could keep using the phone for 90 days-if I am still having problems after I get that, I could sell the phone and probably still come out ahead due to the $250 back that I got, and I could go back to using my 8Plus that I hadn't sold yet.
I would be so disappointed to give it up as I think it's an absolutely fantastic phone and I love the advanced camera. It makes no sense why this is happening! I used my 8Plus recording a slo mo video of the screen to see if I can see flicker like I can on my wife's XS just checking to see if somehow they implemented PWM on this year's LCD screen- but there is no flicker at all, so that is definitely not it.
So.... as noted my last hope is the privacy screen protector. Has anyone else tried that to see if it would help?