That's funny- I also tend to keep my screen at max brightness most of the time, no auto brightness, but I can't say that it seems more dim to me. I was thinking though that on the dim end it does seem darker to me.
This JUST happened to me, I noticed my screen was dimmer than usual at the highest brightness. Saw that it was set to auto brightness. Disabled it but still was way too dim at full brightness setting on the slider. So I did the 3x screen and low and behold it was set to low light mode. How does this happen? did I someone accidentally enable it? Thanks Matheus!