Yea, the internal command is not M500, I forget what it is though. It’s a M10** command.
Edit, found it from here: How to do a larger than 3 x 3 matrix level
You end with a G1029S, G1029D0
Regardless, M503 reads the state of the EEPROM. After a calibration, cross check the output of M503 with M420 V, which is the currently loaded mesh.
I’m suspecting your EEPROM chip is damaged?