I know other people are having this issue what it really annoying as I have only had my watch a few month. Now I tried this morning 30 mins while connected to my phone drained 16% battery turned off the Bluetooth another 30 mins and it did not budge. So the problem is some thing on the update that has caused the battery to drain only while connected to the phone. Let's hope Samsung hurry and fix the issue.
In my findings, I've found that the Hand Washing app drains the battery, you have to uninstall both the watch face, and the app, and after doing that, my battery use returned to "normal". That app keeps the microphone open to listen for the sound of the water to identify when you're washing your hands.
After the latest update on my Galaxy Watch GT it would not last even for a day although it used to last 5 to 6 day.
This is what i did and resolved the issue.
Took a backup from the Galaxy Wear App.
Then reset to the watch.
Connect again from the beginning.
Restored the backup again from the app and everything returned back to normal.