Altimeter, Barometer, Elevation Gain / Loss Issue with the Forerunner 955

I've seen the opposite - typically Garmin Connect's elevation correction adds to total elevation gain/loss, with the watch's altimeter being more accurate. Though if the two are off by hundreds of feet that sounds like a potential sensor issue.

Manually calibrating at the start of an activity, my watch's altimeter has always been well within the 50 foot accuracy number stated by Garmin (though I also haven't done any activities with more than around 1000 feet total ascent). I've found DEM calibration to not be that accurate. The best way to calibrate at the start of an activity is to know the elevation of your starting point beforehand and enter in that value manually.

Suggest using Auto-Cal with Auto sensor mode. In perfectly static weather conditions it is better to have auto-cal off; however, I've found that the altimeter tends to drift in one direction starting within the first hour of activity. Auto-cal might erroneously adjust sometimes, but it will more often than not prevent the altimeter from continuously drifting if the pressure is changing during your activity. This is especially true if your route passes through the calibration point multiple times.

I've only ever used Auto sensor mode. Barometer only is for activities that you know have zero elevation gain/loss (e.g. playing basketball, running on a track, etc.).

I have a similar problem. At the beginning of the activity, even after entering the initial altitude, the altimeter calibrates for a long time and the altitude lags behind me by perhaps 300 m even on the plain. Then it starts to slowly decrease and finally shows a value similar to the GPS altitude. It looks like the altimeter calibrates at the start of each activity.
I tried turning auto calibration on and off and nothing helped...

I've not adjusted any of my settings so just assumed my elevation gain/loss was correct. Anyway a couple of weeks ago I did a 25k run which showed both on my watch and GC as 1,536m ascent (1,542 descent). I'm training for a 3,300m gain marathon in March so wanted to double check this was correct as it's important for training. So I downloaded the GPX file and uploaded it as a route to All Trails. Anyway, according to that it was actually 1,161m gain which is a pretty significant difference!

I don't really want to faff around with settings right before each hill run and it's got me questioning the quality of the data collected (it's far from ideal to training at a -30% deficit.

This looks like a slightly different calculation that generates the deviation. But I have climbed 700m and descended only 300m on the circular route. At the same time, according to gpx, there are 300m climbed and descended. At the beginning of each route, the watch counts 30 to 500m more than it calibrates.

But what probably bothers me the most is that at the beginning of each route it shows me a steep climb even on the level, and thus gives me a bad indication of the performance when running. Instead of 400 watts, sometimes even 1000 watts.

UPDATE

I just got off of the phone with Garmin support.  It was about a 5 minute call.  I gave the individual permission to look in my Garmin Connect data and directed them to my first example (2096 feet ascent from my altimeter; 1455 feet ascent with elevation correction on the same activity in Garmin Connect).  They looked at one example out of the many I had and said, "Ahhh, yup!  I know what's going on!"  After that, they said they could send me a replacement.

I was definitely grateful, though, I wanted to know if I had gotten a watch with a faulty or malfunctioning altimeter/barometer (which it sounds like that is the case by their reaction on the phone).  When I asked this afterwards, they said they couldn't say for sure.  It's probably Garmin's policy not to say what the issue is.  At this point, it seems all I can do is hope the replacement doesn't do this.  

Also unfortunately, they will not have a replacement black Forerunner 955 for 1 month or longer.