I have an Edge 1000 and use that for cycling but the HRV stress readings are all over the place and high during the activity on the 645.
When doing an activity you should not be recording the stress, and if you log the cycle on the 645 it will stop recording.
So my question for Garmin Joey is surely it makes sense for Garmin Connect to realise your Edge has you cycling for 6 hours and to then remove that period from the HRV stress recordings?
My way of fixing at the moment is to record cycling on both devices.