Ciao ​@Demer. Have you ever experienced this issue? Have you tried to reinstall the app on your phone and on your watch (if you have it installed)?

Anyway you can correct the elevation for your past activities from the three dots menu on the upper right part of the activity screen. 

Its the same data on web and app. And yes I have reinstall the app.

I sync also from polar to Trailforks, and in trailforks the elevation seems normal on the same rides 

Ciao ​@Demer. Have you been able to correct the past activities?