Auto fly with a heading of 0

how come that when the navigator publishes a position_setpoint_triplet_s with changed latitude but same longitude results in a heading of around 15°, instead of my desired 0° (fw plane)?
It’s exact when i want to fly east or west (same latitude), heading is then 90° respectively -90°, but with north and south is shifted by 15° to east.

So for example:
waypoint_from_heading_and_distance(lat, lon, 0.0f, 10000.0f, &lat, &lon)
publish that, and my plane flies with yaw approx 15°.