Altimeter continuous calibration via GPS

I've posted a workaround of sorts here https://forums.garmin.com/showthread.php?371978-F5-Barometer-settings-modes-issues&p=990395#post990395 , i.e. : displaying GPS Elevation and (Barometric) Elevation side by side and calibrating the altitude manually by using a "hotkey" for widgets, you can then go to the altitude widget and recalibrate from there, either by entering the altitude manually or using GPS altitude. Have found it to work but haven't checked the impact on D+/D-.

About "Automatic Calibration", according to the F5 manual http://www8.garmin.com/manuals/webhelp/fenix5/EN-US/GUID-C734846-01A7-4F33-86D4-DFBDBC06CDB4.html it occurs "at your GPS starting point by default". But I'm not finding that to happen each time. Has anyone figured out if it depends on the actual offset or possibly the quality of the GPS signal ? On my FR935 I tried setting my altitude to 1042 meters instead of the correct 42 meters and started an activity with GPS Elevation showing 38 meters, but nothing happens, odd.

I did a hike about 10mi with about 4700' ascent, when I reached the summit the elevation in my Fenix was off by almost 400' (Low). Am I expecting too much from the Fenix by wanting it to be spot on or at least within 50ft? I understand when the GPS is turned off and maybe for certain activities, but for other activities like Hiking or mountaineering elevation is much more important.

This is exactly my experience with BP altitude and why for these cases I prefer GPS altitude. Garmin doesn't seem interested in dealing with this appropriately. On my Epix, you can select Elevation and GPS Elevation fields so when I'm hiking or trail running in the mountains, I'll show both. GPS Elevation is usually within a few feet at the top where as BP elevation is often off by more than 300'. The trouble is the weather is so variable from valleys to the tops of mountains that it can't adjust.

I did a hike about 10mi with about 4700' ascent, when I reached the summit the elevation in my Fenix was off by almost 400' (Low). Am I expecting too much from the Fenix by wanting it to be spot on or at least within 50ft? I understand when the GPS is turned off and maybe for certain activities, but for other activities like Hiking or mountaineering elevation is much more important.

This means the weather (pressure) changed between the beginning and the end of your hike, not unusual. See the post just above yours for some tips on how to handle this.

I've posted a workaround of sorts here https://forums.garmin.com/showthread.php?371978-F5-Barometer-settings-modes-issues&p=990395#post990395 , i.e. : displaying GPS Elevation and (Barometric) Elevation side by side and calibrating the altitude manually by using a "hotkey" for widgets,

Auto-climb is helpful here. I have it enabled, and on my Auto-climb screen I have four fields: Ascent, (Barometric) Elevation, GPS Elevation, Vertical Speed.

This means the weather (pressure) changed between the beginning and the end of your hike, not unusual.

No, that's probably not the reason. A change in weather won't cause the barometric elevation to change by 400' in a couple of hours. Not unless there's a major hurricane passing through.

Glajda's explanation is the correct one:

It's actually not so much about barometer drift, which is usually not that high, but temperature compensation and associated difference in altitude.
The watch always assumes changes happen in the standard atmosphere. If it's warmer or colder than standard atmosphere, then the actual change in altitude is different. That's why in winter time when you reach the mountain summit, wour watch can indicate a few hundred meters more, and when you get back to the start, it's correct again.