Why is my Apple Watch Series 9 battery draining so fast?

Watch series 9 w/ cellular running watchOS 11.3.1, paired to an iPhone XR iOS 18.3

Bought it brand new around 2 months ago had excellent battery life for the first few weeks and then suddenly it started draining around 10% per hour. Its been draining like that for the last 3 weeks. Currently I have to charge it around 2-3 times a day its unusable. So far I have tried:

1. Rebooting multiple times ( pressing the crown + side button)
2. Updating my phone
3. Updating my watch
4. Unpairing and erasing (Twice)
5. Resetting sync data
6. Turning off live activities, always on display and low power mode

The battery health is still 100% according to the settings app so I am out of ideas.

Would greatly appreciate any suggestions because I can't get any official support from apple because my watch is a cellular model and the authorised apple repair shop in my city said they cannot file claims on cellular watches.

[Re-Titled by Moderator]