Ascend shows altitude instead (19.21)

When starting last ride the device auto-calibrated altitude and set that as ascend so I started riding with 270 m. 30 km later I had 290 m and this was reported in various places like Strava etc. Anyone else noticed this?

Edit: this is the activity. This area is flat as pancake so there's no way to get 287 m of ascend, more like 20 m.

connect.garmin.com/.../1