Altimeter problem

I'm guessing a bit here, but you may be confusing barometric pressure (weather predictor) with altimeter (GPS info).  My altimeter info is pretty stable when not moving. 

I guess calibration is set to 'automatic', not favouring barometer over altimeter - thus barometer fixed, altimeter variable?

I am not confusing, but I think the Epix 2 might. On my fenix 6 Pro the altimeter worked pretty good, especially last month. Yesterday the baro-pressure dropped 30 hPa, and started rising again at the time I re-calibrated the altimeter in Epix 2.

During the night the baro-pressure has increased again with 35 hPa, and the Epix 2 altitude has dropped further, now reading 28 meters, should be 96 meters which I "DEM"-calibrated to.

So maybe the altimeter in Epix 2 gets confused with large variations in baro-pressure. One millibar equals about 8 meters in altutide. The variations I see are not that big, so the Epix 2 probably uses more variables to detect altitude change. Re-calibartion with GPS solves it, of course, but then it starts drifting.

Auto-calibration "ON", and Sensormode "Automatic".

Never change those settings on the Fenix 6 Pro, so have not changed them in the Epix 2 either.

Outside of an activity with GPS, you aren't going to get continuous calibration regardless of your "auto calibration" settings.  So if the BP is changing, this is exactly what you'd expect to have happen.  It'll calibrate every night at 2am (you can see the bump if you look at your elevation graph when you first get up), but not continuously during the day.  

It may also try to calibrate at other times based on the connection to GCM but I'm not sure on that one.  That would require location services enabled for GCM. 

You are absolutely right and not imagining anything, don't let anyone tell you otherwise.

Garmin pushed an 'improved' algorithm for barometer / altimeter calibration across their whole watch line some weeks ago. There were many negative comments regarding this functionality in the Fenix beta, but Garmin went ahead anyway.

I have written a post on this, including some links:

forums.garmin.com/.../the-terrible-altimeter-calibration-change-from-15-00-16-00-to-16-50-once-again-shows-everything-that-is-wrong-with-garmin-updated-same-problem-on-fenix

I don't understand how the automatic altitude calibration works. When activity starts, the altitude data does not match the DEM map, in order to correct the data, I have to do a manual DEM calibration.

Have not tested that on Epix 2, but the Fenix 6 Pro behaved identical as you describe above. Hopefully Garmin will fix this issue also.

Seems like it works as intended then. Last 4-5 days been pretty stable at correct altitude.

So apparently, it is when large fluctuations i high/low barometric-pressure during a day it will drift far off altitude, unless beeing used in an activity which will re-calibrate.

On "normal" weather-days it seems to be within limits.

I am seeing some altimeter issues too. I have been doing manual GPS calibrations for elevation at the start of each workout. I did a 10-mile run yesterday and passed through the same point twice, once at the beginning of the run and once at the end of the run, with about 1 hour between them. The first time it reported an elevation of 192', and the second it reported an elevation of 224'. This is a significant error in the space of an hour, and it's systematic (the overall elevation the end of the run was clearly higher despite it being a loop and coming back to the same starting point). I just checked the weather data for the NWS station near me, and the pressure did drop slightly from the first time point to the second (from 30.26 in Hg to 30.22 in Hg) so that would be consistent with the watch using barometric pressure to infer elevation.

Would turning off the barometer and having the watch rely on GPS only address this? What is the point of using barometric pressure for elevation when GPS is so accurate these days and we know that barometric pressure can vary significantly with weather?