What I see right now is a kind of confusing.which tells me in turn that the process is not understood which rather indicates master data issues than technical issues.
- MRP does not create POs.
- fixed vendor indicator in source list is not considered during MRP
Hence the entire part of the discussion about auto PO indicator is useless and has nothing do with the case that a source is not assigned when running MRP
bring some facts (screenshots) as evidence that your dates and master data setup is correct.
Present the relevant screenshots from your source list, from material master, from stock requirements list, etc.