Venu 2 heart rate BPM way too low during intensive exercise.

Each activity profile is designed specifically to be the most accurate during the activities that are most similar to them. Always use an activity profile on the watch that most closely matches the activity you are performing. For example, using the Elliptical app while running outdoors could result in incorrect heart rate being recorded. Using the correct profile will help with heart rate accuracy during the activity and for all day tracking, as heart rate is calculated differently for each activity type.

I have this problem too. I don't normally set my watch to record an activity, normally because I'm recording said activity on zwift or my bike computer for example), I expect it to pick up my heart rate regardless.

I noticed today that while my heart rate strap had my bpm at 145 my Venu said 90bpm. when I turned on broadcast heart rate mode on my watch it instantly shot up to match that of my strap (145bpm) 

when I turned off broadcast it dropped back to 90 odd again. 

this feels like a bug tbh, if my watch is going to track my heart rate then it shouldn't need me to start an activity to display an accurate reading

Thanks for your input, I agree that we shouldn't need to start an activity on the watch to accurately monitor the HR, I never needed to do this on the Fitbit I used for years and the Garmin Venu 2 is a much better smart watch so. It's disappointing and I agree it feels like a bug to me.

This appears to be a recurring issue for me, it's been fine previously, I do remember an older version of the firmware had a similar problem and I've only noticed again recently. 

I understand your thought process and comparison to another product. I can only speak to the way our device is intended to function.  The following resources go into more detail. The information I provided previously came from the first link

Garmin Watch Optical Heart Rate Accuracy Tips

Garmin Health Sciences - Heart Rate Monitoring

This is a well-known issue with Garmin. If you're using Zwift, stick to a traditional HR chest strap - it's far more accurate and has no delays.

Yeah, I do use an hrm strap for zwift, however intensity minutes are seemingly only calculated from garmin hardware (bike comp or watch) and while an hrm sorts out zwift nicely, my watch reporting an incorrect value means the connect site does not register any intensity, in fact it seems to think I've been pootlng around at 80bpm rather that the 160 odd my hrm is recording. It's just a poor show that a watch designed to read my heart rate flat out doesn't read a representative value 

I hear you, but I doubt Garmin has any intention of debugging this issue. I've been in the Garmin ecosystem since 2010 and with Zwift since 2014. Over the years, I've encountered all kinds of bugs. It's incredibly frustrating, so I just continue using an HRM strap.

HR is very low when broadcasting to Tacx.

Why would I have to start a cycling activity on the watch, when I'm using Tacx? And why do I need to use a HR strap when I have a watch unless the watch is faulty and not doing it's job!

And why does it matter which activity I'm using that Garmin records it differently. My heart is my heart it does't matter if I cycle or run or fly.....I think Garmin-Tracy's explanation is just a coverup for a bad metric on top of bad software.

An HR strap is widely regarded as the gold standard for heart rate accuracy.
The combination of hardware (e.g., processor, optical sensor) and the laggy Garmin OS leads to discrepancies in heart rate data. The Venu 2 uses the older Elevate 4 optical sensor. 

www.dcrainmaker.com/2021/04/garmin-venu2s-smartwatch.html

According to Garmin, the Elevate 5 sensor offers enhanced optical heart rate monitoring with better responsiveness during various activities.