Copter losing alt even though i have lidar

Hi guys.
i have an s900 frame with a pixhawk flashed with arducopter 3.5.7. using the defaults pids it worked quite stable and i added a LidarLite v3 connected over i2c
i am getting the readings from it perfectly, but the issue i faced was like this.
using mission planner i armed the copter, and issues take off command, i set the height to 5 meters from mission planner.
then i gave it a point to go to, in guided mode. it went there holding the alt quite well,then i gave it another point to go to, it slowly descended from 5 meters to almost 1 meters for no apparent reason.

Does anyone have a clue on what could be happening? Thanks

i have tried attaching the log but i kept getting file size limit message so i uploaded it here:

2018-10-04 19-31-05.log.param (13.2 KB)