Heart rate on Epix 2 activity WAY TOO LOW

https://ph.garmin.com/minisite/garmin-technology/wearable-science/heart-rate/

support.garmin.com/.../

Truly sorry buy that article does not address any of the scenarios outlined above. As I mentioned above, when I do not chose the running activity but just measure my heart rate while running without recording the actual activity, the heart rate is very accurate. It’s when I choose the activity that suddenly it drops by 40 or 50 beats and starts fluctuating all over the place. Put differently nothing to do without external factors as these same factors are present when I don’t record the activity. Or also put differently, the activity algorithm makes the heart rate reading worse, not better. 

And surely, I can understand HR struggling with sports like tennis, boxing, etc, but a HR jumping 40 or 50 up and down while running? If the watch HR monitor can’t get running right, what’s the point? 

Did you try with a soft reset? Hold top left button until the watch turns off and then turn it on again.

Is the watch updated? 

Yeah we both reset our watch multiple times and we are both on latest software version 7.35

Truly sorry but the articles do cover the scenarios outlined in your original post. Pay particular attention to the wording where the two different methods that the WHR uses are explained. Note also the factors that can affect the accuracy of WHR during activity.

This has been discussed ad nauseam over the years since Garmin introduced Optical Heart Rate to its watches. 

Just to summarize again, before someone else explains me again that optical heart rate monitors are less accurate (i think everyone knows this by now). The issue is not that: it reads heart rate well during strenuous activities when NOT recording the activity as an activity. But as soon as you record the strenuous activity as an activity (pick any activity, even running activity), heart rate suddenly drops to 50 bpm below. 

And thank you everyone for help of course, guess I’m letting my frustration get the best of me. 

That's really weird.
If there are complaints about accuracy it's the opposite(when not in activity ).
If a soft reset didn't do anything i would go with a factory reset(somewhere within system settings) but i guess that the best is just to contact Garmin support.
It could be that it's something related to the activity profiles and that they can just send you some files to replace something within your watches.

That does seem weird, it's a long shot but try going in to your heart rate zones and fiddling around in there. Try changing it to %hrr (or whatever), save then change it back. Doing this *may* help kick it out of some strange bug that it's backed itself in to for some reason. Try just changing things and saving, something might update a value somewhere which is then used in some calculation for the HR monitor reading/display.

Try removing any custom heart rate zones for activities and also switch any other heart rate related settings off then back on again.

It seems odd that this would have any effect on the reporting of your actual heart rate but I've seen weirder bugs in my life 

Lastly, give the 8.16 beta a go. Seems pretty solid on mine.