I use Gaia to create routes and export via GPX. I import the GPX as a track into Garmin Explore and the elevation information shows up - screen shot below. When I select "go" on the Garmin explore android app it sends the track to my Fenix 6x Sapphire and all elevation information is gone/missing. All other data is correct, just missing the elevation information.
Garmin Explore on my my Android:
Fenix 6x activity started and no elevation: