Yaw keeps moving even when vehicle is stationary

My rover cannot detect yaw correctly; disarmed, stationary, but yaw keeps decreasing (or increasing, sometimes).
How do I correct that?

Dataflash log is here: dataflash_log.zip

  • ArduRover 3.5.1 with CubeBlack + Here2
  • Use EKF3
  • Accel / compass calibrated