Of course will SAP check all mandatory fields, this is always done for any change, even manual changes.
You probably missed to tell SAP that other fields do not need to be changed. This is usually done through the NODATA sign '/' .
more convenient for such a little change of a single field is the BAPI which I explained in LSMW Material master by BAPI method - Part 1
or even the new MM17: New upload function in MM17 and MASS transaction