just an idea.. take a look at what third party apps if any are installed, often apps installed or used on iPhone are automatically installed on watch and unknowingly these sometimes are the culprits. uninstall them on the watch .
i literally have no third party apps installed just default apple apps on a Ultra 2 . mine varies from 3% to 10% drain overnight on latest beta whilst i wear mine during sleep
give the watch a reboot
what version of watch?
what is the battery health?
what version of software?