Crashing in the middle of exercise (going back to the watch face) is a bug reported long time ago and until now probably hasn't been fixed (my guess is that it may be simply unfixable). I experienced that several times when my Ionic was working (on any type of activity). Usually, it happened soon after starting a workout so no much lost (5-10min). Later, another issue showed up which was random restarting during exercise as well as during doing nothing particular. I recall, I noticed it first time after update that came up last January. I don't know if issue has been fixed because my Ionic died few months later. It could be the latter issue causing problem. You are just not "lucky" enough to see the whole restart.
