Pixracer Barometer

I have a PixRacer (possibly a clone - I’m not quite sure) and am confused with the Data Flash logs Barometric altitude figures.
If I flash the unit with the PX4 stack, I get altitude readings corresponding to my actual height above mean sea level.
If I flash the unit with the APM stack (regardless of flavor copter, plane etc) I always get altitude readings vary close to zero and which loosely follow the inverse of the temperature reading of the sensor.
Any advice on how to troubleshoot here?
Many thanks,

Not sure where your getting these reading from. APM gives altitude above takeoff. So that would be relative altitude not sea level. If you look at the GPS values it gives both Relative and Sea Level altitude.