I had something similar happen to me too. The workout indicator was on the screen and the workout app's complication showed the running animation. But the Workout app didn't have any workout going. Tried to start a new workout and end it, but the indicator and animation was still there. Restarting the watch solved it for me. Hasn't happened again.
Also one thing to keep in mind is that if you start a workout, turn off the watch before stopping the workout and then turn on the watch, the workout will automatically continue. I once got a 20min workout because I started a workout to test something but the app crashed immediately. I had the force restart the device to get out of it because it didn't respond to any button presses or anything else. 20minutes later I looked at the screen only to notice that the workout was still running.