The altitude information is no more available on my activities on Garmin connect since 2weeks now.
It is a bug in the recent version, disabling the GPS Elevation Corrections by default at watches without a barometric altimeter, although is should be on, at activities of those devices. Report it to the Support.
More details at: https://forums.garmin.com/apps-software/mobile-apps-web/f/garmin-connect-mobile-ios/309380/forerunner-35-not-showing-elevation-gain-when-sync-with-garmin-connect
Thanks you for your response. I will report it
