To be fair, your app is not the only one having issues with continuous HR monitoring, I have tested a lot of apps that try cont HR and the same thing happens...they work for a while in the background then just stop monitoring. Seems like the background service/job scheduler is being killed by the battery optimisations introduced in H, the irony being in system developer options there is a list of apps that are exempt from the battery optimisations i.e. Mobvoi's own health apps and Google system apps...however, unlike full blown android on a mobile it seems no apps can be added to the exemption list without requiring some sort of root access I would believe!
With latest update, it seems this issue got better. Now the heart rate sensor can functioning for more than 1 hour, however, I still have this problem once after 2.5h of activity recording with Google fit.
Not sure if HT2 is affect this or not but the Heart rate sensor does not just stopped around 30 min after the latest update. (before it can never last more than 30 min)
With previous version, disable the service (via phone HT2 app) seems help a bit. (The HR sensor lasts longer (~30min) then if the service is not disabled.(15~20 min))
Maybe your latest update does mitigate this problem but if possible, please look into this issue....
Thanks!