I’m also having this issue on my Forerunner 955. I have to end the workout manually.
Did you ever find a solution? I just moved from the FR220 to the 255 and this has been an issue with all my programmed workouts so far on the new watch. I had no issues with the decade old watch.
After today's update to version 13.21, the problem still persists.
My workaround: I add a short time step (about 5 seconds) at the end of the workout.