Dissapointing experience

I hate that one feature would make you dislike your experience so much. Here are a few options:

1. It seems like the technology is not where you'd like it to be. Remember, these watches are still using (mostly) the same software as other watches, it's the hardware that is a premium. If this is an issue, buy an analogue watch instead with a cheaper sports watch you can use your belt with.
2. Don't pay attention to the problems that bother you. As you yourself noted, Garmin continues to make improvements. 
3. Make your own smart watch that is perfect in every way. :-)

Neither of the three points applies on my case.

1. Marq athlete worked ok for eleven months. I can understand that the belt during a 100km cycling is more accurate and it is not a problem for me. But having a watch showing you 40bps during calm situation while watches at 1/3 of marq athlete cost shows the correct value is dissapointing. Remember it seems to happen after an upgrade but doing hard reset , partial solved the issue. But we do not get 40bps but 55bps while the correct value sees to be on 70s.

I will give a last chance to garmin support to fix or replace it.

2. Do not need to comment on this. If you claim to have a sportwatch and ignore heart rate measure , you do not have a sportwatch.

I would wager it's not a problem of your watch but your physiology and how you are built (your wrist).  I find the optical sensor useless for any real workout tracking (same with previous fenix/forerunner generations I've had).  It works better for some people and worse for others.  For my wife it tends to be generally accurate (except at higher intensity workouts).  However, for tracking sleep, resting heart rate or body battery, it's good.  I had the WHOOP device a while ago and I had the same issue issue with optical HR inaccuracy.  You're just not going to beat a chest strap that measures electrical activity of your heart with an optical sensor shining a light on your wrist.  I've had my Marq athlete for 2 yrs now and it's the best watch I've owned with some limitations (resolution and optical HR for workouts).  Maybe try putting the watch on your other wrist for a day or two and see if the data is similar?

Perhaps I was not clear on my initial message. 

I know the limitation of the optical sensors and Marq worked ok for a year given these limitation,

Due to firmware update it stopped working this way and started having serious differences with the correct measurements.

The latest update is that Garmin service reprogrammed the sensor and now it seems that it is working on 99% of the performance I had before. I will keep an eye on it now since I lost trust on the device.

:/.  Good luck! As they say - first world problems with a $2k watch…

1. It's quite common for people to think a firmware/software update has caused a problem when in reality it is just a coincidence.  Hardware failures in something are more common than many people think. This particularly applies to wearables even if the Marq is more robust than most. 

2. Although dismiss his answer, user33 is quite correct to point out the possibility that something has changed with you which makes previously reliable measurements now less so. Again, this could be coincidental to a firmware update. 

3. I think you've got different answers to those you were expecting by linking your problem with the luxury nature of the watch.  It is nothing to do with that. I have a £200 MTB which has taken 20 years of daily abuse in all weathers with only 2 visits to the bike shop vs my £4000 gravel bike, which seems to go there every other month.

You have exactly the same sensors on the Marq as all but the newest Fenix 6 watches so if it was related to a software update, you would see it there also.  As we are not, it is more than likely related to something specific to you, such as a setting problem or a hardware problem.  You are likely right to be contacting Garmin for a solution.