Error in mission mode with USV

While performing the mission mode in Q ground control, the error is happening as shown in the figure.
The airframe is USV, Firmware is PX4, and it is just an simulation mode (SITL)

The figure shows that USV loiters around the second waypoint just after passing the first waypoint. It denotes the speed is about 4m/s, so I redused it to 1m/s in other simulation to make it turns well.

Here I have a question. Is speed of the airframe is the only problem? or is there any other case that happens similar with this and sort of solution? It would be thankful if you guys reply about this issue.