MavProxy reports incorrect altitude with constant bias

I am using MavProxy 1.8.50.

I noticed that the reporting altitude is always 5 meters higher than the real altitude.

However the 'Alt' field in menu bar is correct.

Commands to reproduce:
> mode GUIDED
> arm throttle
> takeoff 10 (reporting 'height 15')
> position 0 0 -10 (reporting 'height 25')


Start by reproducing the GUIDED takeoff and position commands described in the issue, then compare the reported height with the menu bar's Alt field. Trace where these two altitude values are produced; done means the reporting height no longer has the stated 5-meter bias while the Alt field remains correct.

Written by the indexing model from the issue text.