RTL by Low Battery voltage go to a wrong direction

ArduCopter Copter 3.2
1

My APM 2.8 uses internal compass. The Stabilize and loiter mode are OK. And when I switch to RTL manually, it works well, too.
But, just like the topic, when RTL triggered by Low Battery voltage comes, I can’t control the copter by RC(I know) when it flied away from home direction.

2

I mean RTL by Battery Failsafe.

3

I got it.

The compass internal / external maybe the reason.