Since Apple issued watchOS 5.3 I have seen several users report their Apple Watch has been draining battery faster, I have always advised people to restart their devices after an update as this fixes most bugs. We have looked further into this and I have found that more and more users are experiencing battery drain.
Personally I don’t tend to keep an eye on my battery percentage of my watch as it always lasts me a day but given all the reports I have watched it over the last 72 hours and I have noticed despite doing the usual reboots etc my battery is draining much faster. I have personally ensured both my watch and iPhone are running the latest public versions (no beta’s) and the problem persists.
The only thing I can advise is for everyone affected by battery drain to log it to apple using their form, if enough people report it they should look into the issue and provide a fix.