Altitude drop during Auto missions

Hi team, recently I am encountering a problem regarding altitude drop during auto missions. The following details regarding the Hexacopter;
FC: cube orange plus
GPS: here 3+
Hobby wing X6+ motors
purpose: spraying drone
I have enabled the downward facing rangefinder but also I’m facing this altitude drop behaviour. here is the video link and log. Please help me out!

video
log

Has the vehicle been properly configured and tuned using the ArduPilot methodic configurator software instructions?

If not, try that. It explicitly addresses this very altitude issue.