I have the same problem. Activities past March 1, 2015 show what appears to be just a starting point.
I opened up the KML file in notepad++ and there's a huge list of points for the LineString, but all of them are the same point of lat,lon,0 . All lat/lon in the file are the same beginning point for the Laps and Track Points too.
On the Garmin Connect map the activities show up as they should, with a track showing covered.