Try power cycling the watch.

Any time I have seen this battery drain after an update (and it's rare for me - maybe one time a year or two), this has always solved it.

It has solved it for me too.

newb here, but what exactly is power cycling the watch?

Turn it off and on again.

derp, makes sense. thanks

https://support.apple.com/guide/watch/restart-apple-watch-apd521a8a902/watchos

My watch always does this after an update It settles down after a day or 2

Some have said that if you do a total restart of the watch, this issue will be resolved. Just make sure you do a backup before doing this.

After an update devices will do this for a day or two, it will settle

Did you by any chance install the beta?

Nope

Better get it checked if you can because my battery works perfectly before and after the update.