GND_ALT_OFFSET not working

I am currently having an issue with maintaining a constant altitude during long missions (30+ minutes)
I always verify that I am near zero for my home altitude before starting a new mission. I often notice the quadcopter gradually loose altitude as the mission progresses even though the HUD display shows the correct altitude except for the lidar which decreases alt over time. When I land it often says it is 1-3m above ground even though it has landed. Im pretty sure this has to do with the long duration of the flight and changing barometric pressure.According to other users on this forum a workaround this issue is done by chaning the GND_ALT_OFFSET in the PARAMS during flight. Except nothing happens once the new offset is uploaded.

Is this a bug in the firmware?
or am I just doing this wrong.