My only explanation is that the ProMotion, hence the GPU, decided to consume more. It’s evident on how YouTube and the games depleted 30% of the battery while the iPhone 15 Plus was just chill.
And that seems like a problem on both Pro Max. Videos and Games on 120Hz means low battery life. I wonder how it would have lasted ProMotion was disabled and keep the brightened at the same nits.
I guess one could switch on low power mode to limit it to 60 Hz. Not very hopeful though because I've read of 13/14 Pro users getting crazy lag when on low power.