Baro ALT Copter vs Plane - on APM:Plane Baro alt goes crazy

I have technical question on dev team:
When I have loaded Plane version of APM (last version 3.3.0), the Altitude reported in MissionPlanner jumps in range ±50 meters. Whereby when loaded last version of Copter software, the range is in -+ 0.3 meter. The same hardware, the same libraries (AP_Baro, barometer.update() is called in both case) but the altitude in plane version goes crazy outside of normal range.
Unfortunately I have not much time to try debug the code (hope it is possible) to find the problem in code.
Therefore a’m afraid to use it in my plane whereas in my quad copter it works perfect.
I tried also older versions of APM but with the same result. ACM in all present versions on that hardware performed well.
Any clue?

Please provide tlogs and/or dataflash logs to help troubleshoot your issue.

I’ll upload log on next week.

The log will be great.
Just to clarify you loaded the copter firmware on exactly the same pixhawk that’s in your plane to check to baro readings? Or do you have 2 pixhawks, one in the plane and one on the copter and you are comparing the two?
Thanks, Grant