Altimeter calibration bug - can I downgrade the firmware?

You can if you have the 7.80 firmware file. Downgrading requires full reset.

Instructions can be found from beta firmwares and latest might also contain the 7.80.

https://www8.garmin.com/support/download_details.jsp?id=14934 

No risk that I know, but the full reset is PIA. So I would not do it. I would just try to press 15s the power button of that will fix your problems, it usually fixes some weird problems after FW update.

Hello JP, I'm sorry to hear about that; could you go into a little more detail as to what you are experiencing please? For example:

What indicated there might be an issue with the auto calibration? Was there weird elevation data on an activity? Did you try to calibrate the watch and the response didn't make sense?

Is there anything you can do to reproduce the issue?

How long have you noticed this?

I’ve auto calibrated my watch (using DEM option since I can’t get a GPS lock when trying it) and for a few days or so it maintains the correct elevation for my location (~780 ft). Yet today, it’s reading 683 ft. How’s that possible? 

So, I’ve calibrated it again just now (777 ft), and we’ll see how long it holds. 

Hi, thanks for your feedback.

I'm currently at 90meters, everytme I start a training (or even during the day) the altimeter goes sometimes at -4100m for some minutes.

I tried to fully wipe the watch (twice), auto calibrate dozens of times (I even wrote to forerunner-beta mail with details).

I'd be more than happy to continue the sharing with you because I love my watch but I don't like this bug

You'll see an example on the weird behaviour hereafter.

Looking forward for the next steps.

in my case, the altitude is weird.... -8000 - 4500 then 2000 or 4500 or 8000. Tried to calibrate manually by gps and nothing happened (the gps is always ok or very near ok at the sea level where a ilve). 

Hi, same here. Since 8.20 update the altitude calibration is no longer working, always display the message about GPS signal. And the actual information is 300 meters lower than reality. The CPE file show as current.

After a sensor software update   (deployed yesterday) it look is now fixed.

Not fixed for me, still very erratic. Reading -105 metres this morning. 

I had the same bug and I tried the downgrade. I am back to 7.80 and sensor hub 9.0.1. I initially thought it was better as when I was just calibrating the watch at least it was considering the new altitude instead of discarding it right away ...
Finally an hour or so later I moved from 467m to above 4000m. After the upgrade it keept going up as opposed to before where I was getting sometimes -1000m below sea level. 

This random altitude is impacting a lot of features. My run have random ascents. It also keeps saying me I am acclimated to high altitude. It also impact Vo2Max numbers ... 

Of course it happens the week the watch is getting out of warranty ... 

Even after a new full reset of everything it does not work. It stayed at 467m for a few minutes and now is above 20000m