Quadcopter flew away PIX4 1.5.5 on PIXHAWK LITE

My copter with PIX4 stack on PIXHAWK lost control and flied-away the day before yesterday!
the situation was like this:
1, I was actually testing the battery life(before that, several days before, I have flew it successfully , even in AUTO mode, and tested RC fail RTL) so, I let the copter to loiter above ground about 10 meters, in POS-CONTROL mode.
2, during the loiter, I noticed the copter drifting on yaw detection, about 6 minutes later, it already turn to left about 30 degrees.
3, then I noticed the copter was rising and drifting slowly, I started to adjust the position using RC,
4, then to my surprise, the copter didn’t compounded to the RC, at least I remember, even I pull the throttle to bottom position, the copter continued to rise.
5, be afraid of the copter flying toward the city, I turned off the RC , hoping trigger RTL, I have tested this RC
lost FALESAVE several times, and I armed the copter in POSCTRL mode, the GPS locked with HDOC less then 1m,
so, I was hoping it will return to launch.
6, unfortunately, the copter took a turn and fly toward the sea very rapidly, disappeared from my view in several seconds.

I believe the battery can only support less than 5 minutes further flying, so I suppose the copter crashed into the see very possibly.

it is fortunate the copter flew toward the sea, but not back toward the city.

What’s wrong!