The battery percent from my Galaxy S8 doesn’t regularly update on watchfaces. It will update for a little while then stop. It seems that when it stops, if I open the Facer app on my phone the watch will update my phone battery info.
This problem has been going on for quite some time now with no fix.
I have occasionally had a notification pop up in my notification bar saying that Facer is connected to my watch but then the notification goes away. It seems that if this notification would stay keeping the app running in the background the battery info might update regularly.