It happens not only me, but also to a running partner who uses Forerunner 965.
So, basically, we always have a workout with: Warmup step, before a X minutes Run step...
It happens like 3 times in last 2 months...
On those 3 times, it always crash/reboot during Warmup step, when it's close to finish it...
Garmin-Sierra you have permission to contact me