Losing Altitude in Auto Mode

Hello everyone, I would like to share a strange problem that I have been dealing with for a while. When my plane is in auto mode, it starts to lose altitude automatically at certain altitudes. Altitude loss does not always happen, for example, while flying in auto mode at 70 meters, there is no altitude loss, but if I define a new way point at 40 meters altitude from 70 meters, even if the plane actually reaches 40 meters, it still continues to nose down and loses altitude.

When I checked the des_pitch and pitch it seems good so I think there is no problem in my tuning. I made autotune.

Here is some of my flight at 70m waypoints and 70m to 40m.



I want to know why it is still giving nose down even plane reached altitude already??

Here is my dataflash logs: