Large Gaps in Wrist Based Optical HR Readings

Is this what you guys are talking about? I think it's still recording HR data, it's just glitching in the graph. I've had no issues during runs.

There are few threads around for this but this most closely resemble the issues I am seeing. Whilst cycling I get these HR dropouts. Initially I believe I was not wearing the watch tightly enough around my wrist (I am very skinny so it's not that easy to get a good fit with a larger watch), but my later rides are better in this respect. However I still see a difference in the heart rate overview pages online, but not in the iOS app. I am not sure if this is due to missing data during the ride? Incorrectly reported on the ride details page or a smoothing/rounding of graphs on the overview pages online and in the phone app? I have attached two screenshots displaying the same activity/time period. 

CliffTredway said:

I know it has nothing to do with how I wear my watch because I exchanged my watch for a new one from the retailer I purchased it from and the issue still occurs.

I experienced an issue this morning with the HR suddenly jumping beyond max hr during a steady state run. The watch was tight, but swimming in sweat. I've taken the watch off, wiped my wrist down and reseated the watch firmly. The HR reading was nominal for the rest of that run.

You'll find that the drops and the miss-reads are almost always caused by some environmental factor, be it the way that you're wearing it, the fact your skin is too cold (causing your capillaries in the surface of your skin to contract), or from the sensing being confused as it slips around on a sweaty patch of skin.

Try tightening the watch if you're not getting a reading and try and ensure that the skin is dry(ish).

Looks like that is the case for me. I suspect my wrist it a bit too narrow for this watch, I consider replacing if with the Fenix 5s Plus instead (Amazon purchase). I have no problems whilst running/working/sleeping etc, this only happens when I cycle. This morning I was wearing the band very tight, but the majority of the ride ended up without HR data. 

Id wear the watch with the HRM on the inside of my wrist during rides if that's your issue (the HRM is likely pulling away from the skin as you rotate your wrist to hold the bar). You won't be able to glance down to see your HR (unless you're ok the drops of a racing bike), but it'll make better contact with your skin.