I’ve never had this HR response for my intervals and looking for some knowledge about what causes a spike in HR right at the end of the longer intervals? This was 10-8-6-4-2’ running at ~threshold pace steady on a treadmill.
It would help to align the HR trace with effort. Cadence, for example. I often get a group of irregular heart beats as soon as I back off a hard effort. Those might be seen as a rise in HR, and AFAIK perfectly normal. OR, it could be that you are anticipating the end of the rep and pushing a little harder.