Ah, so I'm not crazy. I notice the same thing, in fact it pointed out that my display has a lot of light bleed in one corner. I never see it at the brightness level I use.The 10.13.4 update caused several issues for me: Now my iMac Pro boots up into OSX with near max brightness every time,
Have you noticed in change any fan noise? That was the first thing I noticed after installing 10.3.4. For the first time I'm noticing the fans when I run processor intensive jobs (Handbrake).