I got a 16 Pro Max earlier this week and I was coming from a 13 Pro Max. I didn't think too much of it then but I did notice that the 16 would feel dimmer than my 13 on regular use.
I put them side by side, with the same wallpaper, same settings (auto brightness and true tone enabled) and set the brightness level at the exact same point and the 16 Pro Max is definitely dimmer and not by a small amout, it is easily identifiable.
I found this thread and one from Reddit and seems a lot of people are reporting the issue, I haven't gone through this thread but it hit me... The new thing of this iPhone screen is it goes down to 1 nit right? I don't know how low the 13 Pro Max goes but that would have changed the scale right? Also, I found that both models do 1000 nits under typical/normal use, the only differece is that HDR highlights go from 1200 on the 13 Pro versus 1600 on the 16 Pro and them outdoor nits go much higher up to 2000 on the 16 which I definitely notice when under those conditions.
So maybe there are defective screens out there but wouldn't we have heard about this at this point? To me it feels like the floor level went down by a significant amount yet the ceiling (regular use) stayed the same which would definitely impact the brightness scale/levels. I'll do more research on this.