Garmin Instinct "remaining ascent" value is always wrong after the activity start. Why?

Hi guys, 

when i start an activity (outdoor hiking) i follow a track who i upload from garmin connect site, at start the altitude profile is always ok and i can see the entire profile, but after some hours (1-2) the profile change, and i can no longer see the correct profile to check how much I have left of ascent and descent. it's really annoying. does anyone know how I can do to prevent this from happening?
thank you