https://userapps.support.sap.com/sap/support/knowledge/en/2526081
2526081 - Source with validity end 9999.12.31 / 9999/12/31 / 31.12.9999 is not picked | SAP...
In SNP heuristic run, a valid source of supply (PPM/PDS or T-Lane) with validy end at 9999.12.31 (or 31.12.9999 or 9999/12/31 - depending on the local date...