The main GPS backend is configured as a Mavlink. GPS data is sent using gps_input_send mavlink command without horizontal velocity data (GPS_INPUT_IGNORE_FLAG_VEL_HORIZ is set).
This flow leads to a situation in which a ground course from the GPS state is not calculated at all. That means that all further use of ground course (as an example inside DCM filter) is incorrect since ground speed contains no valid data (I suppose there is just garbage there).
So should not a ground course be calculated in such case similar as ground speed is, as an example during an update state call inside AHRS