Heart rate halves half way through recorded walk

I bought a 5 plus about a month ago. Mine was absolutely fine until the 26th May where it now displays exactly the same behaviour. About 29 mins into a walk the heart rate drops from the 90-100 range down to 60 then stays between 60-67 fro the rest of the walk regardless of how steep a hill or road i walk up. 

I wonder if this coincided with a software update. I can't remember when i last updated it. 

Yep, same thing here. Walk will start out at 110-120 BPM or so and then drop to 60. No rhyme or reason. Sometimes it's halfway through the walk, other times it's a third of the way. And sometimes it works fine all the way through. I've started wearing my OH1+ armband, but I shouldn't have to for a simple walk. The watch should work as it's supposed to. Fix this, Garmin.

same issue for me (fenix 5x  software 19.10)

After 4 or 5 walks where it worked fine, tonight it went from 120 BPM to 55 BPM in a few steps. This was about 90% of the way through our usual 1.5 mile walk. Sweat was not a factor here as it's comfortable and dry. The watch is on snug but not too tight. Well off my wrist bone. No excuse that I can see for it not working or suddenly losing my heart rate.

With all due respect, when you are serious about HR measurement then you use s chest strap. Handwrist measurement is usefull for resting HR, that's it.

That's BS. This watch is marketed as a sports watch. HR measurement is its primary use. I do use an arm or chest HR strap when I ride because the wrist ones never seem to record really high heart rates. But a simple walking heart rate should not be beyond the capability of this watch. Not at this price. And it works fine 80% of the time. If it loses the ability to consistently track HR, especially when the problem seems to have started right after a firmware upgrade, I'd say there's an issue that Garmin needs to address.

I absolutely agree with this. It gets painful listening to people justifying poor and frankly unacceptable performance. The exact same problem I've had is not ok, it'll never be ok no matter how much people Witter on about 'workarounds'. 

Wearing a chest strap for a walk is not a real thing. 

I bought a tennis racket last week, but it came with no strings. Not to worry I'll use it as a pool cue. 

This sounds to me that the OHR fails if a certain limit is exeeded. If starting walking the HR is low and during the walk the HR rises, especially when walking in the heat and/or steep hills. And if you're on the north hemisphere at May the heat is starting which will increase in the next month. But OHR has systemic problems to measure high HR and therefore HR drops to a very low value, maybe HR/2 or HR/3.

It's only a thought from me that unfortunately wouldn't help you.

The problem with optical HR while walking is that there are two rhythms causing similar readings: your heart rate and your cadence. The watch tries to recognize the cadence with its accelerometer from wrist movements and thus lock on your heart rate. With walking those two rhytms are very close to each other, and often cross (during uphill your heart rate rises while your cadence drops, vice versa for downhill). In all optical HR sensors that easily causes the sensor to occasionally lock onto the wrong rhythm (or half-rhytm, when it looses the heart rate and then tries to re-lock).

I think it's something that has to be accepted. If you do sports like regular running where your heart rate and cadence are different and they don't change rapidly (and especially don't cross each other while changing), optical HR is just fine... with running it also helps that your circulation is elevated, making sensor readings easier.

I know it's sort of counter-intuitive that recording HR while walking is more difficult than while running or while being still. But that's just true.