I am experiencing the same since yesterday, without having changed any settings. I used to have ~60% charge left at the end of the day, but now it's down to 20% at 5pm.
I agree; sending back doesn't make sense. I have experienced similar things before with earlier smartphones that suddenly have changed behavior. Typically this happens after an update (although I haven't figured out yet what), and disappear after another update.
But it's always good to see that apparently I'm not the only one
I rebooted my watch yesterday night (watch 4, 46mm), and now the battery drain seems resolved