Let me relate my own experience. I had flown DJI drones for over a year and lifted off from a new location but similiar to previous areas flown so I did not recalibrate the compass. After getting 11 GPS satellites, I lifted straight up. Everything when normally for 4 seconds until, at 49 feet (15 meters), the drone started to fly away at high speed, in a curled flight path. I attempted to make the drone come down but for the next 20 seconds it orbited erratically but did finally get close enough to the ground to auto land. I got a compass warning only after the erratic flight had begun. This all occurred in a parking lot with an asphalt surface. I latter learned, that before the parking lot was there, this area is where a small mountain of iron ore had been stored for a steel mill for ~ 100 years. Much of that ore would have sunk into the ground over the years and was merely paved over.
Your experience seems similar to mine in that when you were high enough, the material in the ground influencing the compass no longer overloaded the earth's natural magnetic field and at that point it went haywire. I think the experts here have given you the cause of the crash. I do not know DJI's warranty policy in this case. It might be interesting to understand the history of your particular patch of ground, if you do not think that rebar in concrete at your takeoff spot was the issue.
The following is speculation on my part, but I have gotten magnetic interference warnings before prior to taking off from concrete with rebar (and I heed the warning and move take off spots), but I wonder if a defused result from my old iron ore spot might have been interpreted differently by the drone and that explains why I got no warning prior to takeoff.
