There were two 10.15.4 updates. I tried them both on my original MBP16. Updating to the first 10.15.4 version (from 24 March) from 10.15.3 correlated with the time when I discovered my eyes are getting strain. I tried to revert back to 10.15.3 in order to check if the update was the reason (drivers) but failed. It might be with the 10.15.3 I just didn't pay much attention to text during my first 2 days of using MBP16. Then there was another supplemental 10.15.4 update (from April 8) that I tried too. It didn't remove the flickering/haze effect.
Just out of interest have you tried a PRAM reset since 10.15.4? I’ve been able to use mine all week with very little strain or motion sickness. Before I could look at the screen for around 5 minutes and had to shutdown and revert to my 2016 TB MBP