Servo output when FS

What is the rule to set serve output when SBUS RX signal is lost and Arduplane goes into FS mode.
I can see that, I have two channels 4 and 3 set to passthru:
RC input channel 4 goes to Servo 4 output,
RC input channel 6 goes to servo 3 output.
When i turn off a radio, servo channel 4 is set to 1500, but channel 3 keeps the last value.