Altitude of quadplane keep on increasing in fixed wing mode

Hi all, I am testing my fixed wing quad vtol these days. Its my first VTOL fixed wing. I have gone through various documents for flying it. During flight when i gave mission to it, after taking off transition took place but the altitude kept on increasing. The altitude i set in mission was 50 but it kept on increasing then i have to take control in Q_loiter mode to land it.
Kindly help me to able to understand it where i am going long. I am attaching a bin log, i am using orange plus in it with latest firmware.

@Michael_Oborne @Mike_ZY @philip