Why VTOL lose speed during flight route? pixhawk2.1

Today I tested my VTOL Fixed-wing, the wingspan is 2500mm, Autopilot is pixhawk2.1. Our testing mission is auto fly as the preset route. Take off, change multirotor to fixed-wing is ok, but It lost speed and changed to multirotor few seconds. Still, don’t find the reason.

The cruise airspeed setting is 18~22m/s. Switch to multirotor airspeed is 16m/s. below is the flight log.