Elevation is wrong

My watch keeps saying I am at a lower altitude than I really am. Check out this bike ride. In reality, the entire ride is above sea level, some of it over 50 meters above. But the entire ride was registered to be well below sea level. Shouldn't the GPS pick that up when I ride for well over an hour?

https://connect.garmin.com/modern/activity/4967051466

Is there something wrong in the compensation tables for planet geometry (how high the sea level is) for my area?