Mag spikes with MPU9250/ICM20948

We are seeing magnetometer spikes (glitches) in the log data from our pixhawk 2.1 cube. We are using firmware based on 1.8.2.WE see them in flight as well as just sitting on the ground.

Pixhawk 2.1 cube has 3 mags to my understanding:
MAG0 = MPU9250
MAG1 = LSM303D
MAG2 = MPU9250

Below are time plots of the data form each of the sensors. Both MPU9250s show periodic spikes in the data, while the LSM303D does not show any.



We also have tested external magnetometers, and the DROTEK sensor with LIS3MDL shows no spikes in the data, while the HERE2 (with driver for the AK09916) shows spikes similar to the MPU9250. This may make sense as they have an internal ICM20948.

There seems to be something odd with communications on the MPU9250-class sensors. Has anyone else experienced this? I found a post showing the same spikes (https://github.com/PX4/Firmware/issues/3953) and that it may have had something to do with multibyte commands, but I didn’t see how it was resolved.

@Jeremy_Zanzig have you succeed to identify the reason for the spikes? I and some other people have experienced same problem, I’ve described mine there.