It might be worth checking the modes available, perhaps cinema or eyecare modes may help if available. Not a monitor but on our Samsung TV some modes flicker visibly while others don’t. Fiddling around the settings accidentally turned it on for me.

How do I check whether my display is having fine PWM modulation?

Look for an in-depth review of your display model. Just checked for my Asus VG278QR and I found out that the background illumination isn‘t reduced by PWM.

Yep, Asus is pretty good on that. I have their IPS gaming monitors - great for coding too.