First make sure that SBMS can control ON/OFF all charge sources and all Loads.
Then when SBMS0 powers on it will do a zero offset calibration as it can turn all charge sources and all Loads OFF and see the real 0A current value.
After that you should see 0A reading when Loads and Charge sources are OFF the battery current will show some small -0.050A or so current depending on battery voltage that is calculated SBMS0 self consumption.
From here you can apply a +2A charge current to PV shunt and both PV shunt and battery shunt should read 2A (slightly less for battery shunt as is -0.050A self consumption).
The you can also test with 10A or 20A and you can adjust a bit the current shunt values to match your reading with an accurate multimeter but the Riedon Shunts are usually within 1% tolerance fairly accurate.
So is likely your multi meter that is inaccurate or the zero offset calibration was not done correctly.
The PV shunt amplifier can have a offset that you can correct in the SBMS menu.
So calibrate first the battery shunt that has no offset since it can measure current in both direction then with a small current 1 or 2A see what the offset of the PV shunt is and add that in the SBMS menu if it is the case (50% of the time). Some current shunt amplifiers will have small positive offset and some negative offset those with positive offset are automatically calibrated during power up and those with negative offset (50% change to get one of those) require manual calibration if you want super accurate low current measurements.
But battery current shunt even if not calibrated with quality Riedon shunt will be within no more than 1 or 2% of the actual current value.