Part 107 UAS pilot reported a loss of UAV control during flight and a subsequent flyaway. During post flight review the UAS crew learned of a GPS failure within the UAS.

Narrative

We were preparing to fly a LiDAR mission using UgCS near ZZZ; my Visual Observer (VO) created the flight plan on his computer while I assembled and inspected the UAV as shown with training received from the manufacturer at their training event I had attended in Month. When my VO had finished his mission planning; I looked it over to ensure mission safety. My VO had set the mission return home altitude to 250 ft.; and the failsafes were set to 'Do Not Modify'. This means that the failsafes are taken from the UAVs native parameters. My VO powered on the Riegl scanner and performed his own visual inspection of the UAV. Once completed; I then prepared to conduct the flight.I armed and launched the UAV and proceeded to take it to our mission altitude in Position Control Mode (GPS Mode) while verifying input and control (Roll; Pitch; Yaw; Throttle). Once at mission altitude; I placed the UAV into Mission Mode. It proceeded along its first pass without issue; and then turned at the end of its area to scan back towards us as planned. Roughly 5 seconds after the turn; the UAV appeared to make a very abrupt stop; then flew several feet off course. My VO called out the issue and asked if I caused the movement; I informed him I did not. In that time; the UAV had begun to rapidly climb altitude and my VO had informed me his computer showed the UAV had lost GPS completely. I switched the UAV into Altitude Control Mode (it uses barometer and IMU for navigation in place of GPS) to regain control of the UAV. However; its behavior did not alter as it continued to climb and fly in a NE direction away from us. I placed my elevation/yaw gimbal in its full downward position; but the UAV seemed to not respond to any control input. I eventually was able to yaw the UAV towards our home location after minutes of the UAV flying uncontrolled in a very aggressive manner; but I was unable to control its Roll/Pitch. My VO and I checked multiple times to confirm we had connection to the UAV; and we appeared to have a strong connection the entire time as it was still within line of sight. After 7 minutes of attempting to regain any kind of control; the UAV was now at a point where it was very far away; and we were losing signal with our first person camera which gave us our only sense of direction. The battery was reading at 45v which is only slightly above our usual landing voltage of 44.5v. Upon loss of signal from our camera; our controller began losing signal and we could no longer see the UAV.We then got in our truck and drove to the top of the nearest hill in the direction the UAV had gone in hopes of gaining signal to recover the UAV. After spending the rest of the day searching; we were unable to locate it. Given the loss of GPS and also the inability to control the UAV in Manual modes; I believe there was a failure in the flight controller which made the UAV misbehave to a point of us losing it.Upon reviewing telemetry logs from my mission control software; I could see that the UAV was flying in mission mode; had a gps failure; then went to AUTOLAND mode. But at no point did the UAV act as if it were in AUTOLAND mode. Later in the log I could see where it was switching modes as I instructed it to; but its behavior never seemed to alter.