I recently posted this on a different but smaller thread I was following on here. Not sure if someone else has posted these steps already in this thread, but I’m not going to read 38 pages to find out…
Potential fix that seems to have worked for me. I've factory reset my 15 Pro Max and fresh installed all apps manually rather than restore from icloud backup. I heard that there can be problems with icloud backup migrations to a new phone where software gets corrupted. This manual install process was slow and miserable. But, it seems to have fixed my battery drain issues. Phone drained 4% overnight rather than 20%. And lasts a full day now. Battery still doesn’t last as long as my 13 Pro Max did, but it’s much better at least.
For those that dont want to wait for if/when Apple fix this, maybe give this a try. Especially as the 14 day returns window is coming up.
Make a new full phone icloud backup to be safe.
Make individual app backups where possible, like Whatsapp, apps with APIs/keys/config files.
Factory reset iPhone 15x.
Don't restore from backup, but sign into Apple ID when prompted.
Manually install all 3rd party apps from AppStore.
Hope for the best.
My battery drain since doing the steps above