Control with Acceleration setpoints

PX4 Autopilot Multicopter 🛸
1

I’m trying to control a drone with acceleration setpoints, sent via mavlink in offboard mode in SITL.
the code is:

    mavros_msgs::msg::PositionTarget pt;
    pt.coordinate_frame = mavros_msgs::msg::PositionTarget::FRAME_BODY_NED;
    pt.type_mask = mavros_msgs::msg::PositionTarget::IGNORE_PX | mavros_msgs::msg::PositionTarget::IGNORE_PY |
                    mavros_msgs::msg::PositionTarget::IGNORE_PZ | mavros_msgs::msg::PositionTarget::IGNORE_VX |
                    mavros_msgs::msg::PositionTarget::IGNORE_VY | mavros_msgs::msg::PositionTarget::IGNORE_VZ |
                    mavros_msgs::msg::PositionTarget::IGNORE_YAW;        
    pt.acceleration_or_force.x = 0.0;
    pt.acceleration_or_force.y = 0.0;
    pt.acceleration_or_force.z = 0.0;
    pt.yaw_rate = 0.0;

I have tuned MPC_THR_HOVER, so that when all accelerations are 0, the drone is just hovering, this is correct.
But when I set
` pt.acceleration_or_force.x = 5.0;

Instead of accelerating forward, the drone starts to move downwards (quite steep at 45 dergess), loses altitude and crashes.

bokeh_plot (17)
bokeh_plot (17)924×368 32.9 KB

bokeh_plot (18)
bokeh_plot (18)924×440 27.1 KB

bokeh_plot (19)
bokeh_plot (19)924×368 27.4 KB

What can be wrong here, and how to achieve horizontal acceleration?