NST17 Welcome to the forum.
I have recently bit the bullet and bought a Dell U2419H and Intel NUC (NUC8i7BEH) which is running the latest Windows 10 Pro + Latest Intel Graphics Drivers.
I have been using the machine consistently for the last two weeks. I wanted to allow that much time to get over any possible placebo affects and also rigorously use the machine to see what would happen if I used it 8+hours a day. The side effects were quite profound; I noticed after day 1-2 that I was blinking less than usual, my eyes had trouble focusing to distances when going outside, feelings of tunnel-vision, also looking at my reflection showed my pupils were dilated a LOT. I did have tension headaches for the first few days, however this actually subsided. I finally connected my known good desktop to the U2419H 2 days ago, and within 5 minutes I immediately felt relief - the tight band around my head had been released 🙂. I've been on my old desktop for the last 2 days and can now compare and contrast how I felt on the new NUC. It almost felt like I was being drugged, and that's not hyperbole.
The positive thing at least in my experience is that this new Dell monitor doesn't cause much strain, no different to my U2414H I bought a few years ago. It clearly is the OS/Driver causing the issues (in most cases) and not the display.
As to the where/why/how it's happening, I don't know. If anybody here can get a VMWare licence from their employer or find a way to obtain a copy (for evaluation purposes) then it should be straight forward to set up a host/client using PCoIP (which works at the pixel level) and document activity between good/bad machines. There is a log viewer available from one of the VMWare developers which has a section for pixel data, perhaps we can reach out to him and ask if different styles of dithering (spatio-temporal, temporal) can be measured using his tool.
Aside from a very expensive capture setup, it could be another approach to consider.