I have noticed this happen, but it has always happened to me after the Pebble crashes and has to reset itself. At that point, Battery+ forgets hour long the watch has been running and just displays "- - " instead.
For me, the crashes seem to mostly happen with alarm timers. If I add a new timer and change the time around, sometimes the alarms won’t activate & the watch will crash at that exact time instead. Seems there’s an off-by-one bug in there somewhere.