I think this comes down to battery life management. By starting an activity on the watch, you are making it clear you want the better data.
Otherwise, it’s in a more passive mode, that samples less frequently. That’s my understanding.
I’ve always had trouble with chest HR straps. I can’t seem to keep them in the right spot and find them very uncomfortable. I switched to the Wahoo arm strap and have had no problems. The accuracy is good enough for me.
I also engaged with Garmin, at length, about this. I have a Venu 2 that I use to track sleep, steps, walking etc. If I don’t start an activity (eg indoor ride when on the trainer or outdoor when on the bike) the HR data is BS. Start an activity and it pretty much perfectly tracks the Garmin chest strap output that is sent to an Edge unit.
They didn’t explain this to me at all and I was ready to give up and return the watch to them. Then I tried starting an activity on the watch itself, effectively parallel recording the session and bingo, realistic HR numbers on my wrist!
A wrist based meter by definition cannot ever measure s heart rate, it can only measure the pulse. The heart can contract/beat without the contraction leading to a pulse
To be fair, if you’re not in a hospital, anh heart rate monitor is generally for entertainment value ultimately.
There’s times I think my fenix 6 is just broadcasting a random number generator, and there’s times it’s spot on with my polar h10. If I forget my strap on the mtb and just record from my watch, there’s times I’ll go 20 minutes at 75beats and then it goes straight to 150
That’s why I don’t even wear a chest strap.
Heart rate? Who cares!
Maybe try broadcasting your HR to TR directly so that you don‘t have redundant files? That’s what I do and it works perfectly on the 6X. It‘s under Sensor&Accessories / Wrist Heart Rate / Broadcast Heart Rate. I use the watch indoors and outdoors this way now and can‘t be bothered with a chest strap.
That’s what I used to do with the Garmin 235, unfortunately for whatever reason the Instinct Solar Broadcast HR doesn’t appear to provide correct data unless its recording an activity.
Garmin gave me the run-around saying it was due to my wrist being in a certain position on a static bike, and a similar response when I said it did the same thing weight training. I have enough experience to have tested it in multiple scenarios and all lead the same thing of accurate data while recording an activity and awful data when not. I provided the info as feedback and got no further replies 
Wow that sucks. both the fact that the watch isn’t as accurate as your older one and the fact that Garmin was/is ghosting you. Does it change anything if you turn the watch on the inside of your wrist and a bit more on the forearm
No no, it IS as accurate, as long as you’re recording an activity. Location is not the issue, I’m certain its sampling rate/algorithm.
Going back to data quality though…at rest it seems good in normal watch mode. Its just when you start doing high HR exercise that it appears to then be off (unless recording activity).
Yea I meant when broadcasting HR. My bad for wording it poorly
Oh I see, yeah. Its not the end of the world, its just a couple more button presses really. But its the principle that counts! 
You can make it work. Here are some things I’ve found…
1.) If you have a dark complexion you will probably be unsatisfied with the performance of your wrist-based HRM. For instance, if I slide my watch up my wrist to accomodate aero gloves the HRM performance gets really sketchy. I think that is because I am moving the light source off the pale, never-before-exposed-to-the-sun, watch-shaped patch of skin to the tanned patch of skin.
2.) If you want the best data for comparison to past efforts, mark the notch you use so that the contact pressure between the light source and your skin is more consistent.
3.) For best results, wear the watch/monitor uncomfortably tight for the effort.
4.) As I’ve said in the past on this forum, if you go from one exercise to another, it’s gonna jack up your wrist-based HRM for a good 6 to 7 minutes. If you do a hard brick, it’s not unusual for wrist based HRM to spike up for the first 6 to 7 minutes of the run.
I wear a chest HRM (Polar H10) but occasionally look at my watch (Garmin Venu 2) to see how it is tracking. In general the watch appears to be useless indoors when doing any sort of workout. However, outdoors it does a fairly passable job of matching the chest HRM.
For reference I’m as pasty skinned as it is possible to be!
The battery in my HRM-Run died the day I posted this, didn’t realize it so my ride on Tuesday had no HR data which means it didn’t count towards training load in Garmin. Used the Forerunner yesterday and it worked great, replaced the battery in the HRM-Run this morning and I’m a little worried about the o-ring so I might be looking at another option soon.
I will say this is probably the 5th time I’ve replaced this battery over many years and the strap still works. If I do end up getting something else later I’ll try for something rechargeable. I’m already charging my bike computer, radar and headlight after every ride so it would be no problem to add one more thing.
The Vivioactive Optical HR sensor is pretty poor, always reads way under a strap by 30-40bpm normally, but I’ve never ran it in the record activity mode. Perhaps it’ll do better then but I prefer to keep my watch as a time piece and not constantly be charging it, as I expect that record activity mode (or whatever its called) will eat the batteries.
Its really not that bad at all (on the Instinct Solar). I use mine to record activities most days and maybe charge it weekly.
I expect it will make all the difference. This is the exact behaviour I see on the other watches.
I have a Forerunner 35 and a 735xt - they only have 2 options - accurate or utter bollocks - with the latter they read about 180-190 for easy runs when my max HR is 174bpm…I obviously use a HR strap when using my 520 plus head unit on the bike…that always reads correctly unless the battery on the strap is failing. Both the forerunners also read accurately with the strap…it’s just that sometimes I forget it when going for a run!
So somewhat related to this thread. I am currently using my Fenix 6x for HR. I broadcast to my Garmin 830. Overall does a good job. My real question/issue is that my phone (pixel 6) will continually try to connect to my watch and when that happens all my peripherals disconnect for a period of time from my head unit (power meter, varia, HR). Does anyone else have this problem, or know how to make the head unit the preferred Bluetooth device?
My workaround is that when I start my ride I just turn off BT on my phone. The only disadvantage is that the live track doesn’t work then.