This keeps happening to me and my SO every now and then. Both of us have the Vivoctive 3 Music. I just checked and out of my last 75 runs 5 had this problem. You can tell from the plot of the pace that it gets a lot smoother after the GPS stops showing on the map. My guess is that the watch resorts to calculating the pace from the steps measured (and maybe extrapolates from the prior data?). Pace and distance mostly seems accurate while I am running. I just notice the failure when I am at home and see the incomplete track marked on the map.
I wish the watch would tell me it lost the signal. No problem pausing the run for a few seconds to catch the signal again really.