Hey scienceboy,
It would be expected behavior for Low Power mode to require manual shut off after being enabled. There would not be an option to automatically turn that off. You can learn more about that here: Use Low Power Mode on your Apple Watch - Apple Support
"Workouts in Low Power Mode
When you use the Workout app in Low Power Mode, your battery lasts longer while metrics such as heart rate and pace are still measured.
You can turn on Low Power Mode in Control Center or Settings, or you can have Low Power Mode turn on automatically:
- Open the Settings app on your Apple Watch.
- Scroll down, then tap Workout.
- Tap Low Power Mode.

Now Low Power Mode will turn on automatically when you begin any workout in the Workout app."
Thanks!