As an update - this appears to be due to cadence lock. For anyone else experiencing it, the watch HR sensor can get confused at the start of a run and use a cadence measurement in place of a HR measurement until it gets a few good readings. The fact that you start running right away w/ a low heart rate and high cadence, low blood pressure, colder temps, looser on wrist, etc, can cause this to happen.
Using a chest strap will eliminate it entirely, or maybe a cadence pod on your shoes, but otherwise ensuring your watch is secured tightly, and perhaps a brief warmup before a workout to get good blood flow, will ameliorate.
EDIT: After playing with this a bit more, a slow takeoff seems to fix this. To be on the safe side, waiting a few seconds until after the run starts to ensure you get a HR lock also gets past this. I was taking off right when the countdown finished and have been running faster lately, so the higher cadence off the line probably was the difference.