I have had a similar experience. My work Lenovo T540 (TN) is totally fine, unless I set the brightness <50%ish where PWM seems to be. Can use it forever on all my screens.
Work got me a Dell Latitude (IPS) 5590 ( 8th gen i5, UHD 620 graphics which is PWM at <99% brightness. Even at 100% brightness it causes me problems, on my good external screens it also causes me problems. I cant use it, I really tried for nearly 2 weeks I used Flux to lower the brightness to avoid the PWM but it still caused me headaches.
My personal machine is a gaming laptop from 2016, 6th gen i7 HD530 (not 520), GTX1060 and its fine. No PWM.
At work before all this lockdown started I got some new monitors as my previous ones were upwards of 10 years old.
I believe they are the U2415, IPS with 6bit panels....on the DELL laptop they're bad, on my T540 I didnt really get to use them for long enough to tell, they are/were very bright though and I hoped I could get used to using them. As they're IPS screens and all my old gear was TN I hope that I just need to get used to them.
But my personal laptop is an IPS screen and I've got it hooked up to my LG 29WL50S-B ultrawide IPS and its fine.
Mine is a different brand but its essentially this laptop as its a Clevo and they're 'modular'