Using only GPS to determine heading

I am currently running APMRover on a pixhawk 4 and using Mission Planner. After disabling all compasses, I noticed that my heading was not matching the GPS track in Mission Planner. This tells me that the pixhawk is using more than just the GPS to determine the heading, even though I have all the compasses disabled. I figured this was likely the IMUs in the pixhawk that are also contributing. How can I make it so that the pixhawk uses ONLY GPS to determine the current heading? I looked quite a bit into how the sensor fusion works with the EKF (I am using EKF3), but with no luck.Thanks!