Hmm, well first of all, thanks for the responses everyone! I didn't expect to see all this discussion when I checked back here. It is interesting to hear some of you don't experience any symptoms when using device with PWM.
Basically, what happened with me is that I bought a ZTE Axon 7 to replace the Lenovo ZUK Z2 I mentioned in the OP. Previously I had used a Moto G, OnePlus One and LG G4 without issues which were all IPS devices without PWM. After getting the ZTE Axon 7 which uses a 240Hz AMOLED panel, I started to experience frequent headaches and eyestrain. It was after a few months that I started to see a connection between using my smartphone and the symptoms getting worse.
I finally stopped using the phone after about 3 months and found that my symptoms stopped immediately. After doing some research I found that the Axon 7 uses the exact same panel as the Samsung Galaxy S7 Edge. And that led me towards lots of complaints online from users of the Galaxy S7 Edge who were experiencing terrible eyestrain and headaches from the exact same AMOLED display.
Eventually I had to sell the Axon 7 because the symptoms were having too much of a bad effect on my health. I switched back to my Lenovo ZUK Z2 in March and haven't experienced any significant eyestrain or headaches since.
But now the time has come to upgrade to another phone, I am worried because I don't want to drop a load of money on a smartphone only to find it causes me the same problems. I like the look of a lot of the Xiaomi devices, but all the IPS models seem to use PWM anywhere from 1,174Hz to 2,315Hz. I notice the Sony Xperia XZ2 is also 2,315Hz like the Xiaomi Pocophone.