Arduplane 3.9 Altitude control not working in Auto flight mode

I have got an easy star type plane with ardupilot (arduplane 3.9) installed on it. I had a flight test. It works well in manual and stabilize mode. but in auto mode it follows the waypoints latitude and longitude correctly in straight motions but when commanded to gain altitude it does not maintain the desired height and comes down slowly and then behaves abnormally. Any comment or suggestion please.

You’ll need to provide a log file.

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