I got my phone on Monday 30th September (delivered at nearly 9pm) and it took me until the following day to transfer everything from my 14 Pro Max, so I didn't start using it properly until Tuesday 1st October. For 2 days, I experienced this issue (excessive battery drain) and I became particularly annoyed because my phone was seemingly permanently attached to a charger and the battery percentage wasn't going up. As of last night I was considering sending it back as other posts on here suggested that Apple support was reporting no issues with the phones even though almost everyone was experiencing the same fault.
I did notice that there is a software update available - and although it is just 18.0 by all accounts this update is just for new iPhone 16's (all variances). It says it's a security update but once installed, my battery life is significantly better - to the extent that at nearly 11.30am, it's at 91% and I've used it for work calls; listening to an audio book for 30 minutes and playing some games since 6.30am when I took it off a normal overnight charge.
Maybe it was the update or maybe something else (although I didn't do or change anything else in my normal phone usage/routine) but the battery drain issue seems to have been resolved.