Are you looking at the widget or the activity after you’ve stopped? Cause I’ve always have had a delay with the widget telling me my current alt. I just chalk that up to the watch needing time to read then graph the current alt. Are you expecting the widget to be real time?
Are you sure that this data is correct? Have you try elevation correction? It's strange cause you would be the only one with post 6.xx fw that Fenix 5 X works fine; have you ever look here : https://forums.garmin.com/forum/on-the-trail/wrist-worn/fenix-5x/1366982-elevation-errors-after-update-to-9-20-from-8-00 ?
I doubt I'm the only one. I'm just trying to understand your use when you say "it doesn't work"
I experienced this yesterday as well while running stairs. The stairs are a known 25m per lap and my watch would often count 0m or 7m and I have no idea why. I returned my 5 because it started acting like this as well. You can see the elevation plot in my run is wonky as well.