Heart rate drops when using exercise app on Sense 2

[Chabit]

The heart rate on my sense 2 tracks really well during brisk walking or quickly running up the stairs or anything that gets my blood pumping. As soon as a select Start Exercise on my device and begin an actual workout, the heart rate tracking fails after a 10 minute warmup.

I've tracked my pulse on my neck and it's almost double what Fitbit is reporting. I rest between sets and intervals, and it still doesn't track my hr correctly. Usually, when I'm done working out is when my hr begins to track correctly.

I had the Sense before the sense 2 and only had issues fairly recently. That's why I bought the sense 2, thinking it would fix the problem. I'm starting to think it's a firmware issue that is t being addressed. And yes, I've factory reset a bunch of times. 

Plz halp

 

Moderator edit: edited title to clarity