FBWI Mode Takeoff Crashed due to Full Pitch [LOG UPLOADED]

We constructed a plane that we believe it is stable aerodynamically. In our first flight, we tried the FBWA mode with Pixhawk the Cube Orange. Before flight, we believed that FBWA mode was only going to assist our pilot, but instead, our RC inputs didn’t work and our plane just climbed with full pitch up! It even reached 88 degrees, despite the rc inputs our pilot sent to make the nose of the plane go down. I’m sharing the log file, please help us undertand what the issue was. We need to try flying again but don’t know if we will face it or not, and we cannot find where the problem is.
NOTE: Center of gravity was properly configured.