Hike activity Altitude issue

Hike today, noticed afterwards altimeter dropped 30m immediately on start and had to be manually calibrated after activity to bring back to correct value. How is this possible assuming watch uses DEM to calibrate when activity starts? Starting location was correct. Calibration durimg activity set to continious, GPS only.

Looks to me like it tried GPS altitude calib on start but no way to tell?

FW 5.10