My Apple Watch battery percentage doesn’t change its always 100%

If the watch is showing 100% charged but shutting down that suggests the charging and battery system is either confused or broken. It should stop charging when it reaches 100% but in this case it is hallucinating and the battery is much lower. You could try unpairing then re-pair to reload all the software, or simply Contact Apple Support and arrange to have your watch checked by Apple.

My series 2 is ok after the update. Have you tried a hard reboot?