Exactly the same issue here, but only since running PB1 on my 7 Plus. I used to put my Watch (Series 0) on charge at night, and it would still have between 40-50% remaining battery, but something about iOS 11 is causing the Watch to drain more during the day and now it's on about 25% when I plug it in. My Watch isn't running a beta OS, so it's something to do with iOS 11. Maybe it's communicating with the Watch more, or demanding more from the Watch. I dunno.
You reminded me about the issue and so I've now submitted feedback to Apple. As I'm sure others have too.
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Also, are you running the iOS 11 beta? That could do it, too.
Seems to be a common issue with the latest iOS 11 beta.