MAV_CMD_NAV_WAYPOINT causes low throttle value

Hello folks,

When I send the below command to ArduPilot on SITL, it also changes throttle value (i.e., 1000) and then decreases altitude of the drone. Even though I can restore a original throttle value by using ‘master.mav.rc_channels_override_send’ command, I am curious about why adding way points changes the throttle value.

master.mav.mission_item_send(master.target_system,master.target_component,
wp_cnt,
mavutil.mavlink.MAV_FRAME_GLOBAL_RELATIVE_ALT,
mavutil.mavlink.MAV_CMD_NAV_WAYPOINT,
2,0,0,0,0,0,
lat,lon,wp_alt)
wp_cnt = wp_cnt + 1

Could you share any idea about this issue?
Thanks in advance.