Radio Failsafe: param RC_FS_TIMEOUT not working

I’m trying to test radio failsafe in SITL according to Radio Failsafe. I’m using ArduCopter for test.

First I change the parameter RC_FS_TIMEOUT to 5 means the RC failsafe should trigger after low throttle for more than 5 seconds, but the RC failsafe triggers immediately when I set rc 3 800. It seems that the parameter RC_FS_TIMEOUT is not working in SITL

I checked the source code in ArduCopter/radio.cpp. It seems that the RC_FS_TIMEOUT only works in function Copter::read_radio() but not in Copter::set_throttle_and_failsafe(uint16_t throttle_pwm) which triggers radio failsafe when throttle value is too low.