Not the original poster, but I've had this happen a few times, too. Interestingly, the daily steps counter stopped, but the step count for the ongoing exercise was still counting, so I had the total number of steps in a day (about 10000) lower than the number of steps made during the single walk on that day (above 11000). And for the record - it wasn't around midnight.
Restarting the phone fixes it, but it's a pretty inconvenient solution during a fitness walk.
