Fenix 5 altitude

If you mean that unwanted change happens during a recorded activity, check you altimeter automatic calibration settings Settings > Sensors & Accessories > Altimeter > Auto Cal.

If the unwanted change happens during "watch mode" (outside an activity) and you want the altimeter to stay constant, check barometer settings Settings > Sensors & Accessories > Barometer > Watch mode. If you set the watch mode to "barometer" instead of "auto", then changes in air pressure are never interpreted as altitude changes.

Downgrade to 14.00 and everything comes back ok

Where can I download the 14.00 version?

www8.garmin.com/.../download_details.jsp

That link is for the latest 16.71 beta and the 15.40 backdate.

I have uploaded the 14.73 beta which includes the 14.00 backdate

Note that downgrading firmware will wipe all the settings/data

https://uptobox.com/uak93fwg1mcx

Sorry use the one including 14.00, yes in that case You must reset everything 

In all fairness I want to point out that I'm on 15.40 now without any problems with altitude. So without a more detail explanation from the original poster it's impossible to know whether downgrading to an older firmware will solve anything. (Version 15.30 had a known bug, which was fixed in 15.40.)

"I'm on 15.40 now without any problems" are you sure ? check out the ABC widget sometime without starting an activity, you can find pressure swings that not even a hurricane would be able to generate

Yes I'm sure, since I reported the pressure swings in 15.30 to garmin with screenshots etc. I haven't seen those in 15.40 in ABC widget or elsewhere. Of course it doesn't prove they don't happen, just that in my use the problems went away with 15.40.