I am also facing a weird complete battery drain overnight for the last two nights and the only different thing I did those two nights is that I kept the Bluetooth on on both my watch and phone. What app are you reffering to?
Have changed my phone to Galaxy S10, and my Galaxy Watch 42mm runs into poor battery life, it does not lasts a full day. Before Galaxy S10 I was using it with HTC and even with bluetooth connect a few times a day it used to last one and half day.
Not sure what it did exactly but choosing the Optimize option on the watch seemed to have fixed the battery drain for me. When you choose Optimize, it wants to turn off connections, lower the brightness, etc. I let it change the brightness (from 7 to 5) but I kept bluetooth and wifi enabled. And I changed the brightness back to 7 almost immediately after the optimization. I'm now back to 3 days without a charge. I took it off the charger Monday morning around 6:45am and I'm still wearing it now, Wednesday night around 7:15pm and I'm at 22%.
Monday morning through Tuesday evening around 7pm, it was down to 49%. It drained 4% overnight so I was at 44% when I put it on around 7:30am this morning.
This is the 42mm Galaxy Watch, btw. It's looking like it's now performing like it was before the One UI update. I had also done a factory reset after the One UI update and that did not fix the issue for me.
I just bought a 42mm. I charged it all night to 100%. 6 hours and 15 minutes later it is at 14%. I barely have any apps on it and I barely no how to use it yet. I will be sending it back. JUNK!!!
I have the Galaxy Watch 46mm LTE version and am consistantly getting 2 full days and with a little less than 10% remaining. Using it as a stand alone device will use a lot of battery life but the biggest factor if it is paired to your phone all the time is brightness. Very happy with the watch, the health freindly messages and all the different watchfaces I can choose from. It amazes me how much tech is in this thing.
I had pretty good battery life, but after I fiddled with some settings it is draining at about 10% an hour. I took mine of the charger this morning at 8am and now, at 4pm, there is only 18% left, where normally (with allways on screen) I am at over 70% by now. I enabled auto back up yesterday and. I have disabled that again now. See if that helps. Do not auto sync music either, is my guess. Switch off anything whereby it is constantly contacting your phone or the internet.