Part 107 UAS pilot reported a malfunction with the UAS that caused a rapid climb above 400 feet AGL. The pilot was able to regain control of the UAS and land safely.

Narrative

Performing demo of two drones flying at around 200 ft; with proper authorization from ZZZ with LAANC. Performed a test 25 minutes before the incident and everything worked as normal with the automated flight plan mission.When performing the real demo; I walked to other side of parking lot in order to maintain VLOS during entire flight. I arm the drone and command to takeoff with the 900 MHz RC link. The drone then goes full throttle for around 30 seconds as I attempt to take control of the aircraft unsuccessfully. I was able to gain control once I cross the parking lot and could see what the telemetry was saying. The GPS vertical position glitched out and the automated mission; along with the Loiter mode(which is GPS based) thought the drone was falling; so it commanded the drone to throttle up to full throttle for around 30 seconds. After 30 seconds of trying to control it in loiter mode; I flipped it into Altitude Hold mode; based on barometer; and then I could see the telemetry vertical speed stabilize. Now that I had control with Alt Hold mode; telemetry feed; and even a live video feed; I could navigate the drone back to being over our parking lot and eventually down to land. looking back at the video footage; the barometer said that it was able 1800 meters once stabilized; and then 1000 meters once landed. Given this; I'd say the drone reached 800 meters AGL.