I knew this is a pretty old issue, but I had encountered this issue as well, after half an hour spent with support online chat, finially got it resovled.
Basically have to go through these steps:
Not ideal but glad my old vivoactive HR watch is still functioning well.
Garmin 745 was stopping recording distance mid way through activities. Tried the on /off/ reset. Tried firmware update in GRMIN express . Tried GPS 'Saturation' (I.e. Waiting for a while before setting off). It was GPS+ Glonass issue. Switched to just GPS for runs and works fine.
Had same issue for 'walks'' and realised you need to change GPS setting for each activity.