Little bit of a bug to report, but could be a one-timer. I set my Ultra to go into low power mode when starting a workout. When the workout ends, low power mode goes off. I do this more as a reminder to end a workout. I have found my strength training workout running behind the scenes all day in the past and low power mode kills the always on display so the black screen works as a reminder for me to kill the workout.
Today my watch got stuck in low power mode even after I ended the workout. It would not let me out of it in the control panel. A soft power off didn't work and it took a hard power down to give me the ability to shut it off. On both restarts, it booted up in pow power mode.