the only thing that works is restarting both the phone and the watch
Have same issue on vivoactive 4 - notification of missed call is unable to be cleared. Just to add, if the same number calls you again and you action the alert (answer/reject) it will clear the notification. Helpful for me as it was a missed call from my wife that got stuck, but perhaps not so helpful to others.
Amazing that this bug is over 5 years old and is still present. Whenever someone calls me for a second (either by mistake or intentionally flash me [that's what Wiktionary says it's called]), call notification gets stuck on my Vivoactive4.
The "easiest" way to clear it I found (without making the person to call me again) is to disable notifications access in Connect app, restart watch and enable notifications back again. Really annoying. Happened to me two times this week already.