Coordinate frame and Fieldtrip import of Neuromag data

I have an issue with coordinate systems when I import a FieldTrip-processed Neurmag MEG. In general I would like to apply SSS. I have a FT-generated .mat file, that has 'neuromag' coord sys of type 'neuromag306'.

When I import it using mne.io.read_raw_fieldtrip, I get sensors' coordinate system to be FIFFV_COORD_HEAD (and MaxFilter does not like it). However if I later save the resulting raw to a file in .fif format and load it again, the sensor coordinate system is now shown as FIFFV_COORD_DEVICE. Even though the values of the 'loc' don't change at all.

I have also noticed that when importing the fieldtrip file, MNE performs some coordinate transformation.

So I guess there is a bug somewhere here -- either coord system is set by MNE wrong when importing or when exporting.

So to get a MaxFilter-compatible coord system, do I have to perform some coord frame conversion, or should I just change the coord system label in the info structure?

MNE version 0.20.3

Start by reproducing the coordinate-frame change with the FieldTrip-generated .mat file and mne.io.read_raw_fieldtrip, then compare the sensor loc values and coordinate labels before and after saving and reloading FIF. Check how the neuromag306 frame is represented in the imported info and whether the resulting data is accepted by MaxFilter; done means the frame handling is correct or the required conversion is clearly documented.