Tree weeks ago I upgraded the firmware to version 7.1.
Everything seemed to be OK after upgrade, but I have NOT checked how correct was the Elevaton and % Grade after the upgrade ...
Two Weeks ago I was on a 300 km brevet with +3550m of elevation and on that brevet started my problems with the barometric altimeter.
Very often on the brevet I was going uphill but the % Grade was showing for some long time some negative percents and the same when going downhill ...
After the brevet my GPS track was with correct length but the Elevation was totally wrong.
I had +3000m elevation but the other colleagues with Garmin Edge 1030 had around 3500 - 3600m.
Few days after that I was back home, started a lot of tests of my Edge 530 and the problems with the Elevation and Grade continued!
Often I was riding on a flat but % Grade was somethimes +15% ... and etc
I searched the forum here, found at least 4-5 similar problems for the last 2 years.
The "solution" for all of them was to return the computer to Garmin and to receive a new one ... 
MY SOLUTION FINDINGS AND FIX:
1. I received my Garmin 2.5 months ago with SW version 7.0
I was unable to find in the forum the link to the 7.0 firmware version - the post is deleted for some reason!?!
I downgraded my firmware to version 6.2 and after that I tested also with version 6.26 Beta which was available in Internet.
The problems with the Elevation and % Grade was much better with this software version, BUT I discovered that now my Elevation and %Grade updates was TOO SLOW.
Garmin 530 needed at least 45+ seconds after the Grade change in order to show the correct Elevation and % Grade.
40+ seconds for update was too too slow especially compared to the operation of the device with version 7.0!
Unfortunately the version 7.0 was unavailable for download anymore!
Fortunately I found the archived version of the forum post from 19 November with the version 7.0 Anons in "The WayBack Machine"
Link here: https://web.archive.org/web/20201119013645/https://forums.garmin.com/sports-fitness/cycling/f/edge-530/246011/new-edge-530-version-7-00-now-available
From there I extracted the correct link and downloaded firmware version 7.0
Hurray!
I installed It and NOW with version 7.0 my Garmin Edge 530 started working OK as before!
The Elevation and % Grade was fast updated - no more than 20 seconds after the Elevation change.
After the upgrade and first successfull test It started raining very hard but I decided to make a second test ... ... ...
And after few minutes the Elevation and % Grade in my Garmin Edge 530 started working very strangely and incorrect again! ... WOW :(
The Grade % was stuck on +1 % but I was on a +3-4%, after 4-5 minutes the Elevation fixed itself to the correct Elevation level and during this time % Grade was for example +15% ... after that again it was stuck on +1% for some time and after that again +15% and etc ...
After I went back home and the device continued to behave very, very strange!
Garmin 530 was on the table but the Elevation was going down with few meters per minute, after some time the Elevation started going up again with few meters per minute change.
I downgraded it firmware for test to version 6.20, after that to 6.26 Beta and 7.1 after that - the problem was the same!
And then luckily I remembered that the tech support in Decathlon said me that water can be problem for the Altimeter ...
I took a hair dryer and dried the device carefully without heating it too much!
I made a full reset + all data deletion with this menu: Setthings / SYSTEM / Delete Data and Reset Settings - I think It's important to delete all old data on the device before upgrade / downgrade if there is some problem like this one ...
After that I installed firmware version 7.0 again.
After 30 minutes after the hair dryer procedure the Elevation Up/DOWN problem stopped and the Elevation normalized!
On the next Morning I tested my Garmin Edge 530 and everything was working perfectly again!!!
CONCLUSIONS:
1. If Garmin works for you, DO NOT upgrade it unnecessarily!
2. My problems with the Elevation and % Grade may not be due to the upgrade to version 7.1, but due to the wetting of the altimeter membrane ... ... ...?
I washed my bike in the yard with a hose. At that moment, my Garmin 530 was on it. So I did not realized at all that at this moment probably I had wet the membrane of the barometric altimeter.
3. Maybe the problems I had was actually due to the upgrade to version 7.1 of the firmware! ...
But I won't risk upgrading again to test it just as long as everything finally works OK! ... :) I stay firm with the perfectly working version 7.0!
4. My Garmin Edge 530 with the old version of firmware version 6.20 and beta version 6.26 work generally OK, but with much slower update of the data from the altimeter.
It takes at least 40+ seconds compared to a less than 20 second update in version 7.0 After I flashed version 7.0 the Elevation and % Grade started working again perfectly corrct and fast!
5. Buy a protective glass for Garmin Edge 530 and a silicone protector.
In Aliexpress it's $ 6-7, but especially the silicone protector largely PROTECTS better the holes on the back of the altimeter membrane from rain.
Link to the missing version 7.0 of the software from the forum:
forums.garmin.com/.../Edge-530-SW-700.zip
Below is a link to my Blog post with detailed description of the problem and my step by step solution findings, translated in English with Google Translate:
https://lupo.thracium.org/kc
