Terrain Radar - Altitude drop

Hello all

I am having hexacopter for an agriculture spraying purposes.I am facing some issues please find below.
Terrain Radar - UAV H30 CAN Microbrain Intelligent
Issues:

  • While moving forward drone there will be no altitude drop if i leave the pitch stick the altitude will gain and drop again.
  • If i switch off the terrain the altitude is maintaining constantly.
    Link:Log
    Please look the log and guide me how to proceed further.