RTL mode the plane speed is gradually decreasing

hi am using pixhawk cube black . am initially tuned my plane , then after i reset the all parameters and again i gave the PID tuning values , then after in FBWA mode i flew the plane at full throttle its going around 30m/s . and some time later i gave a RTL mode it came where am armed and circuling but at circuling time my plane is gradually decreasing the air speed then i take manually and safely land it .
why its decreasing speed when RTL MODE ?.
I give air speed cruise speed is 12m/s and airspeed ratio is 1.98436 ( default values ) .
log file : https://drive.google.com/open?id=1u34mAolxv50vqcauKjaZ2zj7Kay6tZeu