Apple Watch Ultra and iPhone 13 Pro Max battery drain issue

I bought my iphone 13pro max in DEC 2021 and till DEC 2022 the battery health was 100%. I was continuously updating my phone whenever update comes. In DEC 2022, I have purchased iwatch Ultra, and once I paired it was worked fine for 15 days. In JAN 2023, my iphone battery health down and in 15 days it is 96% from 100%. I am shocked.!!!! Now I have turned off Bluetooth in my iphone and using iwatch ultra without paring with my iPhone since Ultra has cellular facility. But again, here some surprises found in iwatch Ultra. The battery of iwatch is draining fast then earlier. I am stressed. Now what to do !!!!! @Apple. Please advise.

[Re-Titled by Moderator]