I have a very similar problem. I have a PC with a monitor which I can use all day without issues. One day, I lend the monitor to a coworker. I told him he could keep it, and I bought a new one (of the same model). When it arrived I instantly felt dizzy by watching at the screen, as if I couldn't focus while I read. I switched monitor with my coworker and voila! I could watch it again all day without symptoms.
Is worth noting that even they were the same model, the first monitor was bought in 2022 and the second one in 2024. So maybe something changed in the panel. But this a prove that it's not only a software problem, it's also a hardware problem.