HMC5843L and Fusion Algorithm

I'm using MPU6050 with magnetometer HMC5843L connected on the secondary I2C, so it is fully controlled by MPU.
I can read magnetometer data within main loop calling function "inv_get_external_sensor_data". Data looks to be correct.

Here i noticed the first strange behavior. I put the sensor on the horizontal plane with Z axis pointing down and I slowly rotate the device.
X magnetometer data keep decreasing slowly while I'm slowing rotating the device. When X reaches 0 magnetometer data stops to be updated so X, Y and Z remain freeze. I keep rotate slowly the device in the same direction and after around 175 degrees X starts again to increase from zero ti its maximum value (215 in my case) while rotating; then starts to decrease to zero then stops for the following 170 degrees. The same is true if MPU is positioned with Z axis point up.

Why magnetometer data are not updated in those 175 degrees?

In my second test I switched from reading magnetometer data to reading quaternion.
Here I have the second strange behavior which - I think - could be a consequence of the previous one.

In my 3D model I set the zero position for the device. If I shake the device with a maximum angle of 50 degrees in any direction (I did not tried with more), zero position is kept correctly. But if I rotate slowly the device on the horizontal plane, after 2 or 3 complete rotations I get an error offset of around 5 degrees with respect original device zero position.

Could anybody explain this?
While shaking fast doesn't have problem while rotating slowly produces absolute positioning errors?

Thanks for any suggestion.
Paolo.