The cause is the screen sharpening algorithm. As you said, iOS 17 is sharper and crisp. This means the screen sharpness has been tuned higher on 17.
I've been using an iPad Pro 2018 (LCD, no PWM) for 6 years. iPadOS 13 gives me eye strain, nausea. But after updating to 16.3.1, I felt totally fine and noticed the content became blurrer than before, which means the sharpness of the screen has been tuned down. However after updating to 17.4.1, I feel nausea again, similar to iPadOS 13, the screen is abnormally sharp and crisp as before.
I assume they use some pixel-wise temporal algorithm to achieve such high screen sharpness. It's aggressive to sensitive persons.
I'm writing feedbacks to Apple now, hoping they could abandon the current way to sharpen screen contents, and fallback to the old way on 16.3.1.