I think this is a bug, not a huge one...I paused an activity with "Resume Later" option. Then tried syncing a new datafield to the watch via the Connect mobile app. Datafield will not sync. Tried about 5 times after disabling/enabling bluetooth etc.
I then went into the paused activity and discarded it. Then tried sync again and it worked. My guess is this is a bug, but not many people will ever see it!