I used the 'resume later' option on a long run this week - or at least tried to! I ran to the local train station, hit 'resume later' and then got the train to the start point of the main run. When I tried to restart, the watch complained that GPS was off, and I needed to select a different activity. There was no way out of this other than to save the current activity and then restart a new one.
Is this a known bug, and any known ways around it?