My elevation data is junky since the last few firmware updates. Is it the updates or my watch?

1. I made the same observation with my watch. I walk on an athletics track and after several laps, the altimeter starts to give wrong values. In addition, the distances are getting shorter and shorter. However, the route is faithful to the track.

I see this elevation craziness when I'm hiking, Garmin support told me it was a once off when I reported so I couldn't be bothered trying to report it again.

Report it in the support chat and insist on a replacement.

I did this and they sent me a "recertified" (new) one in exchange for the broken one for free. Few weeks after warranty ended.

This replacement watch has not shown the described altimeter problems (yet).

I had zero readings plus very high readings on some activities, clean sensor with warm water plus full reset without restoring backup has sorted it for mo for now. 

Sounds sensor related , I have not noticed any elevation issues since last firmware.  If you go up and down stairs for awhile does your 'floor count' go up appropriately ?  How far off is your non-activity elevation when you go to the ABC widget or view current elevation on watch?

Do you shower with your watch on (gets regular cleaning)?

The altitude at the start is exact, but the end is mostly strongly different even if I finish exactly at my starting point.

mince is crazy since 5 months but maybe it's a hardware issue. However seeing that a might not be alone give me hope :) 
Maybe you could try to clean up you barometer ( soak the watch in a water for like 30 minutes or so that it cleans up a bit, didn't help for me  but maybe will help you )