Well like the previous poster, I talked to Garmin, and they deduced the barometric sensor was broken. They are doing a RMA for me. They provided a return shipping label and will send a replacement Fenix 5 (hopefully with a working barometric sensor). A bit of a drag since I had it only a month before it failed. Hopefully the new one doesn't fail.
These have nothing to do with my problem. I don't want it to calibrate automatically during the night. Previous Fenix watches didn't do it. I want it to calibrate at start and during activity. Also, altimeter or barometer mode is a completely different topic.

This morning, as I always do, I drove 22 miles to work in the North west of England. According to several websites there is a difference in altitude between my start and finish points of circa 220 feet, the altimeter on my Fenix 5 shows no difference in altitude at any time in the last 4 hours.

Since the update to 12.20 the altimeter no longer shows correctly. Before the update, I always had a height of 38 meters where I lived. Since the update to 12.20 the altimeter shows 108 meters in the same place. I already calibrated the altimeter via GPS, but it stays at 108 meters, which is definitely too high. Hi, like so many others, my Fenix 5S had the sticky start/stop button problem. I tried the soap/warm water method many a time but it never really solved the problem. Seeing that my watch is out of warranty…. Just outside of warranty, and mine is now so bad, I had to do something. Garmin wants $160 to get a refurb replacement.
The altimeter and barometer readings on my Fenix 6X Pro are suddenly wildly inaccurate. The altimeter fluctuates from subterranean (e.g. -147m) to stratospheric (e.g.12,539m) - I'm at 30m above sea level. Even with calibration manually or via DEM/GPS at the start of an activity or during the day, the elevation changes rapidly.
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garmin fenix 5 plus altimeter issues