Known issue in the WatchAWear community: the on_second(dt) function skips a beat at "00" seconds on the clock. The function "on_second(dt)" updates only between 01 and 59 seconds but does not update at all during the "00" seconds.
Some have suggested a workaround by using on_millisecond(dt) instead, but I believe that function drains the battery faster than on_second.