after updating to 13.21, my 955 sometimes reboots during running. After the watch starts you have a posiibility to resume and continue your activity. But anyway it is a serious bug in 13.21.
I’m on 13.22. This happened to me today. During a run it rebooted about 1 mile in to my 5K race. It tried to resume, but I obviously lost about 1/3 mile.
I was not using any external sensors or headphones - just the watch.