Why is my iPhone 15 Pro's battery draining rapidly after updating to iOS 18.3.1?

Greeting,

I have a question... I have an iPhone 15 pro, it's exactly one year old, my battery health is at 92%... in the last few days my battery is draining rapidly... let's say, today I was talking on the phone, the battery was at 30%, when I hung up the phone it was still 30% and after a few minutes I looked and it says 25% :( then I updated the software to iOS 18.3.1., is it possible that my battery was draining so quickly because I didn't update software?? Otherwise, I pay attention to the battery charge, I try to keep it between 20-80%, I turned off location services and background updates and put the phone in dark mode...

Kind regards,

[Re-Titled by Moderator]