Incomplete Map on Apple Watch workout.

Just wondering if anyone has any advice? Lately when I’ve been doing some walks the map isn’t complete.. For example today it missed about the first 4/5k of a 19k walk in London. It’s been doing this intermittently on some workouts. It’s picked up the distance and stuff just not showing the full route on the map.

I have tried repairing, restoring the watch and both my phone / watch are on latest software updates.

Any ideas what this could be?