Problem with elevation Forerunner 265s

I have already read all problem related topics in Forum, bu to me it seems that my problem is a bit different. 

My fFR265s constantly adds elevation to my activities, also it gives me 2-3x 10 flights of stair climbed in a day, when  I'm nowhere near that. 

Example:

This was simple bike ride, same in both directions. FR gives me 350+ m of elevation, when in reality its closer to 100 (as seen in DEM corrected elevation graph)

Recorded graph:

DEM Correvted graph:

No wind, no change in weather, no water. 

Also, I would go for a run on completely flat circular track and it would give me 70-120m of elevation, which does not make sense. 

This would happen with every activity.

What I already tried:

-Resetting

-Cleaning the barometer port

-Calibrating before every activity

-changing Altimeter settings from "auto" to "Altimeter only" to "barometer only"

No luck.

Is it hardware of software problem?