Flying Lower than Takeoff Altitude -- Uncontrollable Ascent

Hi all, we have noticed an issue when taking off from a higher altitude and then flying to a lower altitude. For example, imagine we are taking off from the top of a hill and flying lower down the hill. In our flight, it looks like the EKF altitude got stuck thinking the drone was below ground, which cause the drone to climb uncontrollably before we had to hit the kill switch. Does anyone have a solution to this problem? I have attached the log below:

https://drive.google.com/file/d/1DFEiVSgASioV_GV5pjjPaz28hFr4lEng/view?usp=sharing

  1. did you use plot.ardupilot.org to analize the log ?
  2. What arducopter FW version?
  3. What vibrartion levels do you have?
  4. did it trigger a falisafe?
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