Flight mode channel 5 is flutuating in mission planner

I am using apm 2.8 ac3.2.1 flysky FS-TH9X 9ch 2.4 ghz transmitter and R8B reciver.i am facing problem during radio callibration.I have set proper pwm value with six position switch on tx and successful get 6 flight modes in mission planner on changing switch postion in transmitter. But whenever i do radio calibration or click on manditory hardware setup and open radio calibration tab i alway saw my ch5 flight mode pwm value is fluctuating.this happen even after succesfull radio calibration. Due to this my flight mode get continously changes.