Wrong HR reading during training when no activity is active on the watch

The HR sensor on Garmin uses a low power when not in an activity. If you want accuracy when doing exercise you need to use and activity as that enables more power to the HR sensors. I

This is one way battery life is extended.

Good question. How long (how many minutes) was your HR higher than 80 when it still displayed 80? If it's less than 1 minute (and even if it's less than 5 maybe)

Another thing I would test is changing to another watch face. It might be a big in it. Was it a Garmin (built in) watchface or one you downloaded from the store?

I use the default watch face from Garmin.

The session lasted 1h 15 min with max HR 184bpm and avg 160bpm.

I checked my Garmin 30 minutes into the training and it was 80, then started the activity for few minutes (that updated the HR to correct value). The HR reading stayed correct for like 20 more minutes, then dropped again. You can see the diagram of my HR from Garmin Connect below. Basically my HR was elevated for the entire duration of the training, peaking at the end to the 184, according Garmin it was 74bpm then (max 164) 

I guess their assumption (and we know what assumptions result in) is that seeing it's an activity watch one would enable an activity when doing same, but your point holds true. Shows how many different variables there are in the way people use their devices. Thinking about it their history came from watches unlikely to be worn all day as well, mainly only for activities. Their HR stuff definitely needs sorting at the moment -to be frank - as there have been so many issues in the past 12 months or so with the OHR reporting and accuracy, which used to be a Garmin strength before a lot of "fluff" was added to the devices.

When i had my AW(s)  , i found the OHR absolutely useless during any activity for me as it used to over-record my HR by miles  - 40 or more bpm - (so I always used my Garmin(s) as well for activities, only in the last 3 years I replaced the AW completely and use the Garmin all the time). The swim tracking was definitely more accurate at picking up different swimming strokes with the AW though.

That's very bad. I'd consider contacting Garmin customer support

About 24/7 HR reliability affecting other stats: Remember that sleep tracking is done only during the night when your arm is still and your HR near or at the resting HR. That's much easier to track than high HR + moving arm. And the HRV status is only measured during the night as well. For the stress reading, it's documented that the watch stops measuring your stress level when it detects you are active (because it's obvious that you are under physiological stress when active, they are trying to measure physiological stress during non-activity).

So regardless of what you think about 24/7 HR reliability during activity (it's not great), you cannot use that to say much about 24/7 HR reliability during rest.

Ok, I have an update:) Today I tested similar situation, this time with two Garmin watches (borrowed Venu 2s from my wife). The reading was pretty much te same for the entire duration of the training on both Garmins and Zwift (see the pic). So something must have affected that very first training, no idea what. I will monitor this but looks like it is not as bad as I thought.