Curve before and after switching is linear, so it does look like a calibration problem.. in this case gain calibration because both curves start nicely from 0 so no offset...
So it it either that meter came with calibration problem, or that one of the shunts was damaged.. since error is large, you can use some other good working multimeter ( no need for high precision ) and see which of the ranges is the one with problem...
Anyways, repair AND/OR calibration is in order.
If it is in warranty, report it immediately...
Regards,
Sinisa