Hello
Regarding the validity issue, take a look on point 2 of the document MRP: Vendor is not selected and general problems on source determination.
You have also mentioned that when the vendor is selected, a different requisition type is used. You should consider that on the MRP group customizing it is possible to define different document types for subcontracting or stock transfer requisitions. So, if the vendor selected is another plant, for example, the document type determined for stock transfer on customizing will be used.
If this information does not help you to solve the issue, please share screenshots of the customizing, source list and the requisition.
Regards
Caetano