I've had this happen a few times now with my only month old Forerunner 255 Music. In the middle of a run I'll look down and the activity has exited to the watch face. If I hit the start button it goes back, the activity is still going, but the distance and pace are way off, obviously not recording while it was booted to the watch face, but the timer is still going. Is it losing GPS signal? The first time it happened I was listening to music from the watch and the music stopped along with this, but the second time I was listening to music through my phone, so it doesn't seem to be the main factor.
It seems like other instances of this I've found on the forum have to do with the watch rebooting during an activity, which isn't what is happening here. Super frustrating for a pretty expensive running tracker to be doing this.